st: Add StDirectionType enum

In order to replace GTK+'s GtkDirectionType. It's bit-compatible with it,
too. All callers have been updated to use it.

This is a purely accessory change in terms of X11 Display usage cleanup,
but helps see better what is left.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/317

js/ui/panel.js

@@ -989,7 +989,7 @@ class Panel extends St.Widget {
 
         menu.toggle();
         if (menu.isOpen)
-            menu.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
+            menu.actor.navigate_focus(null, St.DirectionType.TAB_FORWARD, false);
     }
 
     toggleAppMenu() {