Add an initial Qt6 support

Summary of changes:

1. If Qt5 was not found, fall back to Qt6.
2. Disable scene viewer in the editor when Qt6 is used (osgQt is still based on Qt4 code).
3. Always use a "Grow Screen Limit" editor feature with Qt6 (otherwise it would use the Qt4).
4. Drop outdated Qt5 5.12 support. It allows to use version-less CMake targets to avoid code duplication.

Tested on Windows 10 with such setup:

1. QT_VER="6.4.1" in the before_script.msvc.sh
2. CI/before_script.msvc.sh -k -p Win64 -v 2022 -c RelWithDebInfo

Editor, launcher and wizard build and run with Qt6, but cells view and objects preview are disabled for now.

Possible follow-up work

1. Rewrite osgQt to use Qt5/Qt6
2. Update CI to use Qt6
3. Test !2637 (merged) in the multiscreen setup
4. Test earlier Qt6 versions (at least, 6.2 LTS)