[kwin_wrapper] Transfer ownership of Xwayland socket creation and Xauthority to the wrapper

This way if kwin_wayland crashes we don't need to ensure that new
environment variables need to get synced across the new env.

This fixes an issue where spawning an xwayland application from a
wayland window that survives a crash would fail.

By moving the logic here we no longer need to wait for kwin_wayland to
start before starting plasmashell or even ksmserver as all environment
variables are set. As long as the wrapper is ready we can continue
starting and clients will just block on connect.

That should still allow for both a lot of optimisations both for speed
and cleaning up the startplasma-wayland scripts.

This will be addressed in follow up patches.

Use of kwin_wayland directly with xwayland is still supported for
testing.