window: Parse _GTK_THEME_VARIANT property

Since version 3.0, GTK+ has support for style variants. At the moment,
themes may provide a dark variant, which can be requested by
applications via GtkSettings. The requested variant is exported to
X11 via the _GTK_THEME_VARIANT property - support this property, in
order to pick up the correct style variant in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=645355

src/core/window.c

@@ -195,6 +195,7 @@ meta_window_finalize (GObject *object)
   g_free (window->title);
   g_free (window->icon_name);
   g_free (window->desc);
+  g_free (window->gtk_theme_variant);
 }
 
 static void