cursor-tracker: Add X11 cursor tracker backend

The intention is to use this to actively track the cursor position given
a fixed interval when needed, e.g. when screen casting.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1391

src/backends/meta-cursor-tracker.c

@@ -56,11 +56,6 @@ enum
 
 static GParamSpec *obj_props[N_PROPS];
 
-struct _MetaCursorTracker
-{
-  GObject parent;
-};
-
 typedef struct _MetaCursorTrackerPrivate
 {
   MetaBackend *backend;