main: Move MetaX11DisplayPolicy to a new meta-private-enums.h

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>

src/tests/clutter-test-utils.c

@@ -5,7 +5,7 @@
 #include <clutter/clutter.h>
 
 #include "compositor/meta-plugin-manager.h"
-#include "core/main-private.h"
+#include "core/meta-context-private.h"
 
 typedef struct {
   gpointer dummy_field;