screen: Hide brightness heading when unused

Trying to just hide the widget will cause some severely bizarre
display problems, so just set the heading to be empty instead
for now.

We should probably look at another way of handling this case.

https://bugzilla.gnome.org/show_bug.cgi?id=657606

panels/screen/cc-screen-panel.c

@@ -253,6 +253,9 @@ get_brightness_cb (GObject *source_object, GAsyncResult *res, gpointer user_data
     {
       gtk_widget_hide (WID ("screen_brightness_hscale"));
       gtk_widget_hide (WID ("screen_auto_reduce_checkbutton"));
+      /* FIXME: This needs a proper fix:
+       * https://bugzilla.gnome.org/show_bug.cgi?id=657606 */
+      gtk_label_set_text (GTK_LABEL (WID ("brightness-section-heading")), "");
       g_warning ("Error getting brightness: %s", error->message);
       g_error_free (error);
       return;