Build system Qt6 preparations

I'm not completely sure what I'm doing, but Qt5:: -> Qt::, KF5:: -> KF${KF_MAJOR_VERSION}, some variables in main CMakeLists.txt, and some additional single edits allow Qt6 configuration to go through, without affecting Qt5 compilation.

Some versionless KF variables are not yet in use as they were introduced in 5.89, and plugin .desktop file installation is just ignored with Qt6 for now, but these changes allow starting Qt6 compilation and work on fixing actual build issues there.