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-private.h

@@ -99,6 +99,7 @@ struct _MetaWindow
   char *wm_client_machine;
   char *startup_id;
   char *mutter_hints;
+  char *gtk_theme_variant;
 
   int net_wm_pid;