KF5 branches

[David Edmundson]

There is now a "kf5" branch in all frameworks repos as discussed last
frameworks meeting.

Starting now any commits that are also wanted in the next KF5 should
be cherry-picked after landing.

This does *not* mean master is fully open for going into KF6 dev mode.
Now the kf5 branches exist, we'll set up tooling and infrastructure
and make sure that is all stable. Only after that is all sorted should
master start getting any Qt6-exclusive dev work and version bumps.

Regards

David