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/meta-backend.c

@@ -196,8 +196,6 @@ meta_backend_monitors_changed (MetaBackend *backend)
           priv->is_pointer_position_initialized = TRUE;
         }
     }
-
-  meta_settings_update_ui_scaling_factor (priv->settings);
 }
 
 void