application: activate application when launching a panel

This ensures the control-center window is correctly presented if the
application is already running when the action is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=721969

shell/cc-application.c

@@ -116,6 +116,9 @@ launch_panel_activated (GSimpleAction *action,
       g_error_free (error);
     }
   g_variant_unref (parameters);
+
+  /* Now present the window */
+  g_application_activate (G_APPLICATION (self));
 }
 
 static int