(feat) Library: add 'Prep' playlist marker, add control to append deck track to this playlist

[ronso0]

Allows to quickly add a deck's track to a specific playlist with a controller
_{no, existing options won't do¹}

• add a Playlist sidebar action "Set/Unset 'Prep' playlist"
  -> toggles 'prep playlist' marker (heart icon) for selected playlist
• playlist id is stored in database -> settings -> mixxx.db.model.playlist.PrepPlaylistId

Keyboard shortcut

press Alt+P while sidebar has focus

New controls:

[group],append_deck_track_to_prep_playlist for all decks and samplers
[Library],append_selected_track_to_prep_playlist

Behavior / UX

Testing with a controller revealed that it's handy to also have a "remove" action. For example when you added a track before you notice it doesn't really fit, or takes a strange turn.
To keep it simple, the append...to_prep_playlist controls work for append and remove:

• trigger control
• is track in Prep playlist?
  • NO
    -> add it, show 💙 splashscreen
  • YES
    -> show splash with 💙 with ✔️
    -> trigger again while splash is shown:
    -> remove track and show 💙 (broken heart)

GUI

If there's a demand for a GUI integration (skin button) I'll need to revise this, but for now it's just perfect.

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then it's marked:

Btw works just fine with Sidebar Bookmarks #14508 : )

Let me know what you think about this!

Footnotes

1. 'Add to Auto DJ' is only available in the track menu, not as deck control, and I don't like the extra step of moving tracks from AutoDJ to my prep playlist
   And 'Preparation window' Preparation Window (pannel / view / whatever) + Preparation Lists (or whatever name you like) #14350 is WIP and simply overkill for what I need ↩

[ronso0]

Wow, I've been using this for like 10 sessions now with the ⭐ button on my Kontrol S4 and I love it!

Sometimes I append tracks to early, notice it doesn't fit or takes a turn, and I want to remove it from the prep playlist again.
So I'll try to add a 'remove' control as well.

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edit
I tested (and like) to have just one control:
Is track in Prep playlist?

• NO. add it -> show 💙 splashscreen
• YES
  • show splash with 💙 + ✔️
  • double-press (press again within N ms) to remove -> show 💙

This is way more convenient than having to map add + remove in scripts.
If there's a demand for a GUI integration (skin button) I'll need to revise this (eg. remove the splashscreen), but for now it's just perfect.

Will update this soon.

[ronso0]

Some tests are failing on Win and macOS ARM, but I don't see how they are related to my changes..

Win:
205 - CoverArtCacheTest.loadCoverFromFileRelative (Failed)
206 - CoverArtCacheTest.loadCoverFromFileAbsolute (Failed)
207 - CoverArtUtilTest.extractEmbeddedCover (Failed)
208 - CoverArtUtilTest.searchImage (Failed)
809 - TrackUpdateTest.parseModifiedCleanOnce (Failed)
810 - TrackUpdateTest.parseModifiedCleanAgainSkipCover (Failed)
811 - TrackUpdateTest.parseModifiedCleanAgainUpdateCover (Failed)
812 - TrackUpdateTest.parseModifiedDirtyAgain (Failed)

macOS ARM
78 - BeatsTranslateTest.SimpleTranslateMatch (SEGFAULT)