display: Add a signal for scale changes

This will allows other bits of the UI to monitor scale changes.

panels/display/cc-display-config-dbus.c

@@ -716,6 +716,8 @@ cc_display_monitor_dbus_set_scale (CcDisplayMonitor *pself,
       /* See comment in ensure_gapless() for why we disregard the
          existing layout here. */
       cc_display_config_dbus_make_linear (self->config);
+
+      g_signal_emit_by_name (self, "scale");
     }
 }