Qt6.8: Ensure Mixxx uses "windowsvista" Qt style on Windows

[JoergAtGithub]

• Ensure Mixxx uses "windowsvista" Qt style on Windows, as our skinning looks inconsistent with Qt's "windows" or "windows11" styles
• Add logging of the selected style on all platforms

[daschuer]

Does QT_STYLE_OVERRIDE environment variable still work? I think it should if user needs to fix a corner case.

Do we want to make the 2.5 branch also work with the 2.6 environment? I can imagine that this can be convenient for us developers.

[JoergAtGithub]

No it does not work - and I do not plan to spend work for this env variable, which we never documented.
If you prefer, I retarget this to 2.5.

[daschuer]

Yes. Please Target 2.5.

[daschuer]

This is working for all Qt applications by default:
https://doc.qt.io/qt-6/qapplication.html#QApplication

This code breaks it for windows.

The main issue is that Qt prints also a warning message that a certain style is used for override. This message is wrong if we change it afterwards and should be avoided.

It looks like QApplication::setStyle("windowsvista"); can be set outside QApplication, before MixxxApplication().
This way QT_STYLE_OVERRIDE will still work and everything is fine.