Issue 3889 - Dispatch subscription.downloading event on transfer

lib/synchronizer.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 110 matching lines @@
   {
     let subscription = Subscription.fromURL(downloadable.url);
     subscription.lastCheck = Math.round(downloadable.lastCheck / MILLIS_IN_SECOND);
     subscription.softExpiration = Math.round(downloadable.softExpiration / MILLIS_IN_SECOND);
     subscription.expires = Math.round(downloadable.hardExpiration / MILLIS_IN_SECOND);
   },
 
   _onDownloadStarted: function(downloadable)
   {
     let subscription = Subscription.fromURL(downloadable.url);
-    subscription.downloadStatus = "in_progress";
+    FilterNotifier.triggerListeners("subscription.downloading", subscription);
   },
 
   _onDownloadSuccess: function(downloadable, responseText, errorCallback, redirectCallback)
   {
     let lines = responseText.split(/[\r\n]+/);
     let match = /\[Adblock(?:\s*Plus\s*([\d\.]+)?)?\]/i.exec(lines[0]);
     if (!match)
       return errorCallback("synchronize_invalid_data");
     let minVersion = match[1];
 
@@ skipping 173 matching lines @@
             let data = "[Adblock]\n" + subscription.filters.map((f) => f.text).join("\n");
             redirectCallback("data:text/plain," + encodeURIComponent(data));
           }
         }, false);
         request.send(null);
       }
     }
   },
 };
 Synchronizer.init();