Support callback param for CKEDITOR.editor::setData

[Garry Yao]

In order to comply with the asynchronous nature of 'setData' in wysiwyg mode, the user need to listen for a internal "contentDom" event for the very early access to a fully loaded document, we should provide a simpler form API like editor.setData( data, callback ).
Correspondingly, the 'afterSetData' event which already existed now should be fired right before we invoke this callback function, so user could make their choice between this two approaches.