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.