Adds optional dependency to X11_XCB and gets used in X11WindowedBackend
to create an XLib Display if dependency is present.
This allows to create an EGL backend for the X11WindowedBackend.
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.
