dateMenu: Show indicator for messages that skipped banner mode

The summary may contain notifications that have not been seen by the user
and won't be shown as banner. Currently this is only the case for resident
notifications that are emitted by the focused app, but it will become more
common as we will start limiting the number of queued notifications.
Indicate to the user that more notifications are available by displaying a
small dot in the top bar button.

https://bugzilla.gnome.org/show_bug.cgi?id=744850

js/ui/messageTray.js

@@ -1456,6 +1456,10 @@ const MessageTray = new Lang.Class({
         }
     },
 
+    get queueCount() {
+        return this._notificationQueue.length;
+    },
+
     contains: function(source) {
         return this._sources.has(source);
     },