Interface definition of a factory that returns
NewPage instances. To use a factory, set it in the document editor using
setNewPageFactory(NewPageFactory). The document editor will ask the used factory for a new instance every time
the user wants to add a new page. If the factory is backed by a user interface (for example a dialog) it must itself care for proper retention of its state
across configuration changes, as the document editor wont do this.
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.