viewSelector: emit a signal when the view is empty

This is very a useful point in time when the size of the side controls
can change without affecting any visible view. Emit a signal at that
time.

https://bugzilla.gnome.org/show_bug.cgi?id=693924

js/ui/viewSelector.js

@@ -205,6 +205,8 @@ const ViewSelector = new Lang.Class({
     },
 
     _fadePageIn: function() {
+        this.emit('page-empty');
+
         this._activePage.show();
         Tweener.addTween(this._activePage,
             { opacity: 255,