st: Add StPolicyType enum

In order to replace GTK+'s GtkPolicyType. 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/search.js

@@ -402,7 +402,7 @@ var SearchResults = class {
                                                y_fill: false,
                                                overlay_scrollbars: true,
                                                style_class: 'search-display vfade' });
-        this._scrollView.set_policy(Gtk.PolicyType.NEVER, Gtk.PolicyType.AUTOMATIC);
+        this._scrollView.set_policy(St.PolicyType.NEVER, St.PolicyType.AUTOMATIC);
         this._scrollView.add_actor(scrollChild);
         let action = new Clutter.PanAction({ interpolate: true });
         action.connect('pan', this._onPan.bind(this));