effect chain preset saving/loading, custom effect defaults, and parameter hiding/rearrangement

[Be-ing]

NOTE FOR TESTERS: The new GUI widgets have only been added to Tango so far.

This PR is a continuation of @kshitij98's GSOC project from 2018 (#1705) to implement effect chain preset saving/loading. Since Mixxx 2.1, Mixxx has saved and loaded the state of the effects units to/from an XML file on shutdown/startup. However, due to the overcomplicated architecture, this was tightly coupled to the initialization of the effects system. To repurpose this functionality for saving/loading XML presets while using Mixxx, the entire effects system in the main thread (src/effects) had to be rewritten first. There are minimal changes to the DSP code in the audio thread; the only change to that is removing the useless EffectRack layer.

This branch adds several new features:

• The state of an effect unit can be saved as a preset with a user-specified name. This is done with a new widget in the effects units that shows a popup menu. The presets are saved in the settings folder, in a new effects/chains subfolder.
• Chain presets can be imported and exported as XML files from the preferences.
  
• Effect parameters can be hidden and rearranged. This allows the user to control which parameters are available for manipulation by controllers. Parameters can be hidden/shown from the same menu as is used for chain presets. Parameters can be rearranged by dragging and dropping the parameter names.
• The state of an effect can be saved as a default snapshot that is used whenever that effect is loaded. This is done through the same menu as effect chain presets. Together with parameter hiding and rearrangement, this replaces the legacy LV2 effects preferences added in Mixxx 2.2. The snapshots are saved as XML files in a new effects/defaults subfolder of the user settings folder.
• Chain presets can be loaded to the QuickEffect units with a new combobox widget. This is similar to Traktor's Mixer FX but more powerful because the user can create arbitrary chains of effects with custom meta/superknob linking.
• A handful of default chain presets are included in res/effects. I have designed these so they are useful when loaded into the QuickEffects units which are only controlled by the superknob. If you come up with cool presets, please share! :)
• The lists of effects and chain presets visible in the comboboxes in the skin can be customized from the preferences. There are separate lists of chain presets for the regular effects units and the QuickEffects.
• New load_effect ControlObject for effects that loads an index in the list defined by the user in the preferences.
• On first startup, a chain preset with a single effect is autogenerated for every effect with a default metaknob linking and this is added to the list of QuickEffect chain presets in addition to the chain presets from res/effects. It is not added to the list of chain presets for the regular effects units. The chain preset autogenerated from the Filter effect is the default.
• The old next_chain/prev_chain and chain_selector ControlObjects for effect units cycle through the list of chain presets.
• New load_preset and loaded_preset ControlObjects for effect units. These use an index in the list of chain presets defined by the user in the preferences. This allows controllers to be mapped to loading chain presets without coupling the chain preset with that particular controller; the user can choose which presets to use with the controller by arranging the list in the preferences. This is designed to be used with the Mixer FX buttons on third generation Traktor controllers (Kontrol S2 Mk3, Kontrol S4 Mk3, Kontrol S3). It could also be used with the Color FX buttons on the Pioneer DDJ-SX3, DDJ-1000, and DDJ-800.

To do:

• Reimplement effects saving on shutdown/reloading on restart
• Reimplement hiding of parameters for LV2 effects
• Saving/loading and importing/exporting effect chain presets
• Custom default snapshots for each effect
• Reimplement EffectChainSlot next/prev_chain ControlObjects
• Add ControlObject to load a specific chain preset to an EffectChainSlot
• Allow loading chain presets for QuickEffects
• Load default chain presets from res folder
• ControlObject to indicate which preset is loaded in a chain
• Let users arrange custom order of effects lists
• Reimplement EffectSlot next/prev_effect ControlObjects and add ControlObject to load effect at specific list index
• Separate QuickEffect chain preset list from chain preset list for regular effect chains
• Autogenerate QuickEffect chain preset for each effect with a default metaknob linking
• Drag and drop parameter names for rearrangement
• Write new tests

[Holzhaus]

Can you mark this [WIP] until it's ready to review?

[Be-ing]

Woohoo! This branch is now beyond feature parity with master. 🎉 🎉 It is ready for review and testing. I want to get this merged for 2.4.

You can save and load custom effect chain presets. You can also save the state of an effect as a default snapshot so that any time you load that effect in the future, it will load with that state. This replaces the hacky toggling of effect parameters in the preferences for LV2 effects and it works for built-in effects too. DlgPrefLV2 has been removed. The chain presets as well as effects presets are stored as XML files in a new "effects" folder in the user settings folder. The old effects.xml file is still used and this branch reads old effects.xml files from master, 2.2, or 2.1. All these new features were implemented while a net of 2184 lines of code were removed.

The GUI to access these new features is only a crude proof of concept right now. There is a new button which pops up a menu to save/load chain presets, hide/show parameters, and save default effect presets. I have only added it to Tango for now (when the effect units are expanded).

The main thread parts of the effects system has been completely rewritten, so reviewing by looking at the diff on GitHub would be overwhelming. I suggest checking out this branch locally and exploring the code. I will add documentation soon.

Further features I want to implement are letting the user rearrange the list of chain presets via the preferences. I want to add new ControlObjects to load stored chain presets so controllers can map scrolling through the list. Users could also set up loading an effect chain with a specific button by mapping a button to a position in the chain preset list and arranging the list in the preferences. I want to make chain presets able to be loaded as QuickEffects. When combined, these features will allow fully supporting the Mixer FX section on the NI Kontrol S4 Mk3:

I also want to add importing and exporting of chain presets. It would be fun to ship some default chain presets with Mixxx too. This branch can be merged before any of these features are implemented.

[Be-ing]

I did some file reorganization. I think the new structure is well organized:

be@localhost ~/s/m/s/effects (effects_refactoring) [141]> lsd --tree
  .
├──   backends
│  ├──   builtin
│  │  ├──   autopaneffect.cpp
│  │  ├──   autopaneffect.h
│  │  ├──   balanceeffect.cpp
│  │  ├──   balanceeffect.h
│  │  ├──   bessel4lvmixeqeffect.cpp
│  │  ├──   bessel4lvmixeqeffect.h
│  │  ├──   bessel8lvmixeqeffect.cpp
│  │  ├──   bessel8lvmixeqeffect.h
│  │  ├──   biquadfullkilleqeffect.cpp
│  │  ├──   biquadfullkilleqeffect.h
│  │  ├──   bitcrushereffect.cpp
│  │  ├──   bitcrushereffect.h
│  │  ├──   builtinbackend.cpp
│  │  ├──   builtinbackend.h
│  │  ├──   echoeffect.cpp
│  │  ├──   echoeffect.h
│  │  ├──   equalizer_util.h
│  │  ├──   filtereffect.cpp
│  │  ├──   filtereffect.h
│  │  ├──   flangereffect.cpp
│  │  ├──   flangereffect.h
│  │  ├──   graphiceqeffect.cpp
│  │  ├──   graphiceqeffect.h
│  │  ├──   linkwitzriley8eqeffect.cpp
│  │  ├──   linkwitzriley8eqeffect.h
│  │  ├──   loudnesscontoureffect.cpp
│  │  ├──   loudnesscontoureffect.h
│  │  ├──   lvmixeqbase.h
│  │  ├──   metronomeclick.h
│  │  ├──   metronomeeffect.cpp
│  │  ├──   metronomeeffect.h
│  │  ├──   moogladder4filtereffect.cpp
│  │  ├──   moogladder4filtereffect.h
│  │  ├──   parametriceqeffect.cpp
│  │  ├──   parametriceqeffect.h
│  │  ├──   phasereffect.cpp
│  │  ├──   phasereffect.h
│  │  ├──   reverbeffect.cpp
│  │  ├──   reverbeffect.h
│  │  ├──   threebandbiquadeqeffect.cpp
│  │  ├──   threebandbiquadeqeffect.h
│  │  ├──   tremoloeffect.cpp
│  │  └──   tremoloeffect.h
│  ├──   effectmanifest.cpp
│  ├──   effectmanifest.h
│  ├──   effectmanifestparameter.cpp
│  ├──   effectmanifestparameter.h
│  ├──   effectprocessor.h
│  ├──   effectsbackend.h
│  └──   lv2
│     ├──   lv2backend.cpp
│     ├──   lv2backend.h
│     ├──   lv2effectprocessor.cpp
│     ├──   lv2effectprocessor.h
│     ├──   lv2manifest.cpp
│     └──   lv2manifest.h
├──   defs.h
├──   effectbuttonparameterslot.cpp
├──   effectbuttonparameterslot.h
├──   effectchainslot.cpp
├──   effectchainslot.h
├──   effectknobparameterslot.cpp
├──   effectknobparameterslot.h
├──   effectparameter.cpp
├──   effectparameter.h
├──   effectparameterslotbase.cpp
├──   effectparameterslotbase.h
├──   effectslot.cpp
├──   effectslot.h
├──   effectsmanager.cpp
├──   effectsmanager.h
├──   presets
│  ├──   effectchainpreset.cpp
│  ├──   effectchainpreset.h
│  ├──   effectparameterpreset.cpp
│  ├──   effectparameterpreset.h
│  ├──   effectpreset.cpp
│  ├──   effectpreset.h
│  └──   effectxmlelements.h
├──   specialeffectchainslots.cpp
└──   specialeffectchainslots.h

[Be-ing]

I added a new section to DlgPrefEffects to import, export, rename, and delete effect chain presets:

[Be-ing]

Woohoo, you can now load chain presets for the QuickEffects. I have included two default effect chain presets that are installed with Mixxx. If the effects/chains folder does not exist on Mixxx startup, these default presets are copied there. This allows the user to delete, modify, or rename the presets without forcing them to keep the defaults in their list of presets. By default, the QuickEffects load a special chain preset that only has the Filter effect. Generating this chain preset in EffectsManager rather than shipping an XML file for it allows the "Filter" string to be translated.

I made a new WEffectChainPresetSelector widget as a QComboBox subclass to make this feature available in skins. I was going to stick with configuring this in DlgPrefEQ, but then I saw that Traktor has a combobox in the main window for this and realized that is a better idea to make the feature more discoverable and accessible. Currently I have only added the new widget to Tango as an ugly proof-of-concept. @ronso0, could you test this and give some feedback on the design and/or send me a pull request to improve it? The new section in DlgPrefEffects could use some design work too.

I have also added a new load_preset ControlObject for effect chains that can be mapped on controllers to load a specific effect chain with a button press. Unlike Traktor, the effects loaded are not coupled to the controller mapping. load_preset takes an integer index in the list of available effect chain presets, the order of which is set in DlgPrefEffects. The old next_chain, prev_chain, and chain_selector ControlObjects have been reimplemented and are now useful for the first time.

Next I plan to reimplement next_effect, prev_effect, and effect_selector ControlObjects. I will also add a ControlObject for chains to indicate which preset was last loaded into them. This is required for a feature of the Kontrol S4 Mk3 in which the enable buttons of the Mixer FX knobs are color coded based on what is loaded.

I am aware the EffectsManager class has grown enormous. I will split it up.

[Be-ing]

Still no explanation has been made justifying why this hasn't been merged.