messageTray: Make notification banners unfocusable

Unlike entries in the calendar's message list, banners are not subject
to the normal keynav chain, and making the banner actor itself unfocusable
allows for the focus to be moved to the action area when expanded.

https://bugzilla.gnome.org/show_bug.cgi?id=747205

js/ui/messageTray.js

@@ -508,6 +508,7 @@ const NotificationBanner = new Lang.Class({
     _init: function(notification) {
         this.parent(notification);
 
+        this.actor.can_focus = false;
         this.actor.add_style_class_name('notification-banner');
 
         this._buttonBox = null;