backends: Make meta_backend_update_last_device() take a clutter device

Instead of a pretty x11 specific device ID. This also updates the argument
of the ::last-device-changed signal to be a ClutterInputDevice.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/852

src/core/events.c

@@ -273,10 +273,7 @@ meta_display_handle_event (MetaDisplay        *display,
   source = clutter_event_get_source_device (event);
 
   if (source)
-    {
-      meta_backend_update_last_device (backend,
-                                       clutter_input_device_get_device_id (source));
-    }
+    meta_backend_update_last_device (backend, source);
 
 #ifdef HAVE_WAYLAND
   if (meta_is_wayland_compositor () && event->type == CLUTTER_MOTION)