Allow switching between legacy and new QML UI with command arg

[acolombier]

This change reintroduce the --qml argument to allow switch to QML dynamically. It also refactor slightly QML singletons to allow Qt to create a singleton per QML engines

[JoergAtGithub]

I could compile this on Windows locally. But it asserts on startup (without --qml):

warning [Main] ControlObject "[Controls]" "ShowDurationRemaining" already created
DEBUG ASSERT: "!"pCreatorCO != nullptr, ControlObject already created"" in function class QSharedPointer<class ControlDoublePrivate> __cdecl ControlDoublePrivate::getControl(const class ConfigKey &,class QFlags<enum ControlFlag>,class ControlObject *,bool,bool,bool,double) at C:\Users\Joerg.WORLDWARTWEB\source\repos\JoergAtGithub\mixxx\src\control\control.cpp:175

 	Qt6Core.dll!00007ffbd92e5438()	Unknown
 	Qt6Core.dll!00007ffbd92e9563()	Unknown
>	mixxx.exe!mixxx::`anonymous namespace'::handleMessage(QtMsgType type=QtCriticalMsg, const QMessageLogContext & context={...}, const QString & input={...}) Line 358	C++
 	[External Code]	
 	[Inline Frame] mixxx.exe!mixxx_debug_assert(const char *) Line 9	C++
 	mixxx.exe!ControlDoublePrivate::getControl(const ConfigKey & key={...}, QFlags<enum ControlFlag> flags={...}, ControlObject * pCreatorCO=0x00000157810ec8a0, bool bIgnoreNops=true, bool bTrack=false, bool bPersist=false, double defaultValue=0.0000000000000000) Line 176	C++
 	mixxx.exe!ControlObject::ControlObject(const ConfigKey & key={...}, bool bIgnoreNops, bool bTrack, bool bPersist=false, double defaultValue=0.0000000000000000) Line 24	C++
 	[External Code]	
 	mixxx.exe!DlgPrefDeck::DlgPrefDeck(QWidget * parent, QSharedPointer<ConfigObject<ConfigValue>> pConfig={...}) Line 40	C++
 	mixxx.exe!DlgPreferences::DlgPreferences(std::shared_ptr<mixxx::ScreensaverManager> pScreensaverManager={...}, std::shared_ptr<mixxx::skin::SkinLoader> pSkinLoader={...}, std::shared_ptr<SoundManager> pSoundManager={...}, std::shared_ptr<ControllerManager> pControllerManager={...}, std::shared_ptr<VinylControlManager> pVCManager={...}, std::shared_ptr<EffectsManager> pEffectsManager={...}, std::shared_ptr<SettingsManager> pSettingsManager={...}, std::shared_ptr<Library> pLibrary={...}) Line 180	C++
 	mixxx.exe!MixxxMainWindow::initialize() Line 293	C++
 	[External Code]	
 	mixxx.exe!OpenGLWindow::event(QEvent * pEv=0x0000001cb64fbdf8) Line 67	C++
 	[External Code]	
 	mixxx.exe!`anonymous namespace'::runMixxx(MixxxApplication * pApp=0x0000001cb64ffbd8, const CmdlineArgs & args={...}) Line 76	C++
 	mixxx.exe!main(int argc=1, char * * argv=0x00000157a12d7ed0) Line 206	C++
 	[External Code]	

[daschuer]

I can confirm this is working with Ubuntu Focal

[daschuer]

some comments:

[Holzhaus]

Just tested this on Arch, I can confirm that it works, with the legacy interface it crashes on shutdown:

fatal [Main] ASSERT failure in WWaveformViewer: "Called object is not of the correct type (class destructor may have already run)", file /usr/include/qt6/QtCore/qobjectdefs_impl.h, line 129
Aborted (core dumped)

Unsure if it is caused by this PR or unrelated.

[JoergAtGithub]

@Holzhaus Could you please share the stacktrace.

[acolombier]

I couldn't reproduce on PopOS 22.04. I tried exiting right after the init sequence (if I quit it during the init sequence, it crashes, but I can reproduce that on main too). I also tried to quite after loading a track/waveform, in vain. Perhaps something to do with your OpenGL config?

[ronso0]

@acolombier Please give the latest commit a more meaningful message, for example
cmakelist: order includes alphabetically

[acolombier]

@ronso0 I was planning to squash the commit once everything is working anyway but I will rename it now.

[ywwg]

works for me on Ubuntu 22.04

[daschuer]

The code looks good to me now.
Can you remove the dedicated Ubuntu qml build? Than it is ready to merge for me.
Thank you

[acolombier]

I've rebased the branch and remove the non-QML build for Ubuntu

[daschuer]

Thank you. LGTM