Draft: WIP: Move to kf6

I've been working on this quite a while, and I think it's in good enough shape to get somebody else to look at it. As far as I can tell there are no major problems, but please keep in mind I haven't yet tested everything.

The first half of the commits are required in a bundle because without them it wouldn't compile, later ones just fix bugs/errors.

NOTE: The commit messages a lacking but I'm working through them, but that is time consuming and boring. NOTE2: This work I think generates a new bunch of warnings, especially of the qsizetype vs int, I will try to fix those afterwards, but they don't seem to stop anything from working.

Any comments or reviews are much appreciated and really sorry for the big dump of commits.