Create the factory for returning the given
NewPage instance that should be returned by this factory.
This method is called every time the factory should create a
NewPage instance. Since creation of the new page can be a longer running
asynchronous operation (for example via a page configuration dialog shown to the user) this method receives a
callback that the factory
can call once the new page is ready.
||OnNewPageReadyListener for notifying the editor once the new page is ready.