Make QtWayland::Compositor subclass the generated wl_compositor class

Change-Id: If07c7ca86ef87f503e1e79663beb9b4ddbafd739
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>