Those are stub specialized classes for MetaWindowActor. This will
help ensuring that we do not execute X11-specific code paths on
pure Wayland clients.

The relationship between the window actor and the surface is the
following:

 * Wayland: MetaWindowActorWayland + MetaSurfaceActorWayland
 * X11: MetaWindowActorX11 + MetaSurfaceActorX11
 * Xwayland: MetaWindowActorX11 + MetaSurfaceActorWayland

It is not possible to have MetaWindowActorWayland backed by a
MetaSurfaceActorX11 surface.

!368