Issue 1106 - Add currentVersion Pref and compareVersions API

lib/prefs.js

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2014 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 20 matching lines @@
   subscriptions_fallbackerrors: 5,
   subscriptions_fallbackurl: "https://adblockplus.org/getSubscription?version=%VERSION%&url=%SUBSCRIPTION%&downloadURL=%URL%&error=%ERROR%&channelStatus=%CHANNELSTATUS%&responseStatus=%RESPONSESTATUS%",
   subscriptions_autoupdate: true,
   subscriptions_exceptionsurl: "https://easylist-downloads.adblockplus.org/exceptionrules.txt",
   documentation_link: "https://adblockplus.org/redirect?link=%LINK%&lang=%LANG%",
   update_url_release: "https://update.adblockplus.org/%NAME%/update.json?type=%TYPE%",
   update_url_devbuild: "https://adblockplus.org/devbuilds/%NAME%/update.json?type=%TYPE%",
   update_last_check: 0,
   update_last_error: 0,
   update_soft_expiration: 0,
-  update_hard_expiration: 0
+  update_hard_expiration: 0,
+  currentVersion: "0.0"
 };
 
 let values = Object.create(defaults);
 let path = _fileSystem.resolve("prefs.json");
 let listeners = [];
 let isDirty = false;
 let isSaving = false;
 
 function defineProperty(key)
 {
@@ skipping 69 matching lines @@
     let index = listeners.indexOf(listener);
     if (index >= 0)
       listeners.splice(index, 1);
   },
 };
 
 for (let key in defaults)
   defineProperty(key);
 
 load();