Cleanup X11 code from MetaWindowActor

MetaWindowActor handles sending NET_WM_FRAME* X atoms to clients - even pure Wayland clients. Having it implemented in MetaWindowActor itself is somewhat convoluted, since MetaWindowActor is not tied to any specific display server.

Clean it up by moving the X11-specific code to the newly introduced MetaWindowActorX11.

Merge request reports