backends: add 'monitors-updated-internal' signal to only update internal state

Adding an internal signal and use it to update the internal state before
emitting "monitors-changed" which will be repeated by the screen to the world.

https://bugzilla.gnome.org/show_bug.cgi?id=788860

src/backends/native/meta-cursor-renderer-native.c

@@ -1071,7 +1071,7 @@ meta_cursor_renderer_native_new (MetaBackend *backend)
   priv =
     meta_cursor_renderer_native_get_instance_private (cursor_renderer_native);
 
-  g_signal_connect_object (monitor_manager, "monitors-changed",
+  g_signal_connect_object (monitor_manager, "monitors-changed-internal",
                            G_CALLBACK (on_monitors_changed),
                            cursor_renderer_native, 0);