MetaWindowXwayland derives from MetaWindowX11 to allow for some Xwayland
specific vfunc that wouldn't apply to plain X11 windows, such as
shortcut inhibit routines.
https://bugzilla.gnome.org/show_bug.cgi?id=783342
This is specifically about managing X11 windows, not necessarily
running as an X11 compositor. By that I mean that this code is
still used for XWayland windows, and event handling is still and
modesetting / monitor management is still in core/.
This is also a fairly conservative move. We don't move anything
like screen.c or bell.c in here, even though those are really
only for X11 clients.