Fix expand animation for submenus

Figuring out the size of an unmapped actor is not completely
reliable because styles aren't fully assigned until an actor
is mapped. So show the submenu before computing the size we
want to tween to.

https://bugzilla.gnome.org/show_bug.cgi?id=645949

js/ui/popupMenu.js

@@ -1059,9 +1059,9 @@ PopupSubMenu.prototype = {
         // we don't implement the !animate case because that doesn't
         // currently get used...
 
+        this.actor.show();
         let [naturalHeight, minHeight] = this.actor.get_preferred_height(-1);
         this.actor.height = 0;
-        this.actor.show();
         this.actor._arrow_rotation = this._arrow.rotation_angle_z;
         Tweener.addTween(this.actor,
                          { _arrow_rotation: 90,