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.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/368
