Port all Qt and KF deprecations, cleanup now uneeded code
The conclusion of my KF6/Qt6 preparations for this repository.
@stikonas Could you please give this critical look/round of testing. I am not not sure how to test all of the code I have touched. For the porting I have followed my previous experience, Qt docs and the deprecation messages (both KF and Qt ones).
cc @ahmadsamir