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

src/shell-stack.c

@@ -146,7 +146,7 @@ shell_stack_get_preferred_width (ClutterActor *actor,
 static gboolean
 shell_stack_navigate_focus (StWidget         *widget,
                             ClutterActor     *from,
-                            GtkDirectionType  direction)
+                            StDirectionType   direction)
 {
   ClutterActor *top_actor;