Duplicate DOM event handlers are registered in IE.

[Martin Kou]

To reproduce:

1. Open replacebyclass.html.
2. Type the following code in the URL bar:

   javascript: void((f=function(){alert('hi!');},body=CKEDITOR.document.getBody(),body.on('click',f)));
   

3. Click on the document body, you should see the message "hi!" appear once.
4. Now remove the event handler with the following code in the URL bar:

   javascript: void(body.removeListener('click',f));
   

5. Click on the document body, you should no longer see the "hi!" message.
6. Add the same event handler back to the document body.

   javascript: void(body.on('click',f));
   

7. Click on the document body.
8. Now you have 2 "hi!" messages - the previous event handler wasn't really removed!

This issue is blocking #3074.

[Martin Kou]

domobject.js is calling detachEvent() in IE without adding the 'on' prefix in the event name, and so it did't really remove the event handler.

[Martin Kou]

Fixed with [3197].