st: Remove custom text direction stuff

Clutter has its own built-in system for managing text directions, like GTK+.
Convert over to use this.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

js/ui/workspace.js

@@ -528,7 +528,7 @@ const WindowOverlay = new Lang.Class({
 
         let settings = new Gio.Settings({ schema: BUTTON_LAYOUT_SCHEMA });
         let layout = settings.get_string(BUTTON_LAYOUT_KEY);
-        let rtl = St.Widget.get_default_direction() == St.TextDirection.RTL;
+        let rtl = Clutter.get_default_text_direction() == Clutter.TextDirection.RTL;
 
         let split = layout.split(":");
         let side;