Add "persistence" server capability

This will allow clients to determine that the server has notification persistence.

https://bugzilla.gnome.org/show_bug.cgi?id=631722

js/ui/notificationDaemon.js

@@ -325,6 +325,7 @@ NotificationDaemon.prototype = {
             'body-markup',
             // 'icon-multi',
             'icon-static',
+            'persistence',
             // 'sound',
             'x-gnome-icon-buttons'
         ];