Disable toplevel fixed positions on Wayland

Given that the wayland protocol does not have window positions a recent change to Qt development branch avoids having a mismatch of positions and tells client code the window is at the topleft of the screen. This fixes multiple bugs.

Unfortunately this breaks a few usages within plasma where a side-channel sends that absolute positional information over a bespoke protocol. Currently we proxy this all through QWindow geometry where the new Qt behaviour will throw is askew.

Whilst our longterm plans are to find future options away from the plasmashell protocol we may as well have our Qt6 build work given the patch is trivial and harmless.

It also allows for potential backporting if our software is already secured against issues. Hence the cherry-pick.