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-private.h

@@ -27,6 +27,11 @@
 #include "backends/x11/cm/meta-cursor-sprite-xfixes.h"
 #include "meta/meta-cursor-tracker.h"
 
+struct _MetaCursorTrackerClass
+{
+  GObjectClass parent_class;
+};
+
 gboolean meta_cursor_tracker_handle_xevent (MetaCursorTracker *tracker,
 					    XEvent            *xevent);