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/backends/meta-backend-private.h

@@ -41,6 +41,7 @@
 #include "backends/meta-pointer-constraint.h"
 #include "backends/meta-renderer.h"
 #include "backends/meta-settings-private.h"
+#include "core/meta-context-private.h"
 #include "core/util-private.h"
 
 #define DEFAULT_XKB_RULES_FILE "evdev"

src/backends/meta-backend.c

@@ -66,6 +66,7 @@
 #include "backends/x11/meta-backend-x11.h"
 #include "clutter/clutter-mutter.h"
 #include "clutter/clutter-seat-private.h"
+#include "core/meta-context-private.h"
 #include "meta/main.h"
 #include "meta/meta-backend.h"
 #include "meta/meta-context.h"