Implement languagechange event, fired when accept languages changes.

Source/core/frame/WindowEventHandlers.idl

 /*
  * Copyright (c) 2013, Opera Software ASA. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 20 matching lines @@
 
 [
     ImplementedAs=DOMWindowEventHandlers,
     LegacyTreatAsPartialInterface,
     NoInterfaceObject, // Always used on target of 'implements'
 ] interface WindowEventHandlers {
     //attribute EventHandler onafterprint;
     //attribute EventHandler onbeforeprint;
     attribute EventHandler onbeforeunload;
     attribute EventHandler onhashchange;
+    attribute EventHandler onlanguagechange;
     attribute EventHandler onmessage;
     attribute EventHandler onoffline;
     attribute EventHandler ononline;
     attribute EventHandler onpagehide;
     attribute EventHandler onpageshow;
     attribute EventHandler onpopstate;
     attribute EventHandler onstorage;
     attribute EventHandler onunload;
 };