The overlay window is only needed for the X11 based compositors. Given
that it is better suited in the X11 platform. Unfortunately it is not
possible to completely move it into the platform plugin as it is still
referenced in KWin core (e.g. SceneXRender). Due to that the
OverlayWindow in KWin core is turned into a pure virtual class with the
implementation being moved into the plugin.
The platform API gains a new virtual factory method which is only
implemented in the X11 platform.