As with the compositor type enum, also have the X11 display policy enum,
as it's also effectively part of the context configuration. But as with
the compositor type, move it to a header file for enums only, and since
this is a private one, create a private variant meta-enums.h.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
