As we have specific window types per display server, having it in the parent class makes building without wayland harder to achieve Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2418>
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