This new backend allows to start a kwin_wayland server nested on an
X-Server by using a normal X11 window as output. This allows testing
kwin_wayland without needing to start another Wayland server first.
The behavior is triggered by using new command line arguments:
--windowed
--x11-display=<:0>
With optional --width and --height arguments.
In this mode the WaylandBackend is not created at all.
So far the backend is not fully integrated yet and only the QPainter
backend supports this mode.
