popupMenu: Close menus on session mode changes

We are already closing top bar menus on session mode changes, but
as this behavior makes sense for any other menus as well - dash
context menus or the background menu for instance - just generalize
the behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=787676

js/ui/popupMenu.js

@@ -465,6 +465,7 @@ var PopupMenuBase = new Lang.Class({
 
     _sessionUpdated: function() {
         this._setSettingsVisibility(Main.sessionMode.allowSettings);
+        this.close();
     },
 
     addAction: function(title, callback, icon) {