[Qt6] Use CMake

[Amphaal]

As you may know, the Qt team has changed their policy regarding building tools, and went for CMake instead of their very own qmake for Qt6 (https://www.qt.io/blog/qt-6-build-system).

@Skycoder42, are you yourself familiar with CMake ? Would you consider maintaining CMake building scripts in parallel of qmake ones ?

Cheers !