Issue 5459 - Add support to show a notification based on the number of ads blocked

test/stub-modules/prefs.js

 "use strict";
 
 let listeners = [];
 
 let Prefs = exports.Prefs = {
   enabled: true,
   savestats: true,
   subscriptions_autoupdate: true,
   subscriptions_fallbackerrors: 5,
   subscriptions_fallbackurl: "",
   notificationurl: "http://example.com/notification.json",
   notificationdata: {},
-  notifications_ignoredcategories: []
+  notifications_ignoredcategories: [],
+  blocked_total: 10
 };
 
 for (let key of Object.keys(Prefs))
 {
   let value = Prefs[key];
   Object.defineProperty(Prefs, key, {
     get()
     {
       return value;
     },
@@ skipping 14 matching lines @@
   if (listeners.indexOf(listener) < 0)
     listeners.push(listener);
 };
 
 Prefs.removeListener = function(listener)
 {
   let index = listeners.indexOf(listener);
   if (index >= 0)
     listeners.splice(index, 1);
 };