Issue 4795 - Use modern JavaScript syntax

utils.js

-let backgroundPage = ext.backgroundPage.getWindow();
-let require = backgroundPage.require;
+"use strict";
 
-let Services = backgroundPage.Services;
-let Synchronizer = require("synchronizer").Synchronizer;
-let Utils = require("utils").Utils;
-let Prefs = require("prefs").Prefs;
-let FilterStorage = require("filterStorage").FilterStorage;
-let FilterNotifier = require("filterNotifier").FilterNotifier;
+const {require, Services} = ext.backgroundPage.getWindow();
 
-let subscriptionClasses = require("subscriptionClasses");
-let Subscription = subscriptionClasses.Subscription;
-let DownloadableSubscription = subscriptionClasses.DownloadableSubscription;
-let filterClasses = require("filterClasses");
-let Filter = filterClasses.Filter;
-let BlockingFilter = filterClasses.BlockingFilter;
-let defaultMatcher = require("matcher").defaultMatcher;
+const {Synchronizer} = require("synchronizer");
+const {Utils} = require("utils");
+const {Prefs} = require("prefs");
+const {FilterStorage} = require("filterStorage");
+const {FilterNotifier} = require("filterNotifier");
+
+const {Subscription, DownloadableSubscription} = require("subscriptionClasses");
+const {Filter, BlockingFilter} = require("filterClasses");
+const {defaultMatcher} = require("matcher");
 
 /**
  * Shortcut for document.getElementById(id)
  */
 function E(id)
 {
   return document.getElementById(id);
 }