wayland: Add wl_global filter manager

One can add a wl_global filter to a wl_display instance, which can be
used to decide what clients should see what globals. This has so far
been used to limit a Xwayland specific protocol extension to only
Xwayland. In order to expand the logic about what globals are filtered
to what clients, introduce a filter manager and port the Xwayland
specific protocol filter to this new manager.

Tests are added, using a new dummy protocol, to ensure that filtering is
working as expected.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2810>

src/wayland/meta-wayland-types.h

@@ -73,4 +73,6 @@ typedef struct _MetaXWaylandManager MetaXWaylandManager;
 
 typedef struct _MetaWaylandXdgForeign MetaWaylandXdgForeign;
 
+typedef struct _MetaWaylandFilterManager MetaWaylandFilterManager;
+
 #endif