CKEDITOR.event duplicate dom event registration

[Garry Yao]

Dom event delegation registration is currently incorrect, if the specified event were registered twice, the listener function suffer from duplicate invocation. Reproduce Steps:

1. Open one editor page in FF
2. Execute the following codes through console:

   	var body = CKEDITOR.document.getBody();
   	body.on('click', function(){
   		alert('first');
   	});
   	body.on('click', function(){
   		alert('another');
   	});
   

3. Every listener was invoked twice.