Issue 2375 - Implement "Blocking lists" section in new options page

messageResponder.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 346 matching lines @@
       case "subscriptions.update":
         var subscriptions = message.url ? [Subscription.fromURL(message.url)] : 
                             FilterStorage.subscriptions;
         for (var i = 0; i < subscriptions.length; i++)
         {
           var subscription = subscriptions[i];
           if (subscription instanceof DownloadableSubscription)
             Synchronizer.execute(subscription, true);
         }
         break;
+      case "subscriptions.isDownloading":
+        callback(Synchronizer.isExecuting(message.url));
+        break;
     }
   });
 })(this);