shell-gtk-embed: Lower tray icon windows to the bottom of the stack

Otherwise they break the "top level window" detection used by the
unredirect code in mutter, causes game windows not to be unredirected
when tray icons are present.

https://bugzilla.gnome.org/show_bug.cgi?id=701224

src/shell-gtk-embed.c

@@ -106,6 +106,8 @@ shell_gtk_embed_window_created_cb (MetaDisplay   *display,
                                              0, 0 /* offset x/y */);
       cairo_region_destroy (empty_region);
 
+      gdk_window_lower (gdk_window);
+
       /* Now that we've found the window we don't need to listen for
          new windows anymore */
       g_signal_handler_disconnect (display,