messageTray: add an indicatorCount property

This filters out resident and transient notifications in the normal
case, but just returns the number of unread messages for the Telepathy
implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=687787

js/ui/messageTray.js

@@ -1153,6 +1153,11 @@ const Source = new Lang.Class({
         return this.notifications.length;
     },
 
+    get indicatorCount() {
+        let notifications = this.notifications.filter(function(n) { return !n.isTransient && !n.resident; });
+        return notifications.length;
+    },
+
     get unseenCount() {
         return this.notifications.filter(function(n) { return !n.acknowledged; }).length;
     },