An action method called by the New menu command, this method creates a new
NSDocument object and adds it to the list of such objects managed by the document controller.
Responding to Action Messages
An action method called by the Open menu command, it runs the modal Open panel and, based on the selected filenames, creates one or more
NSDocument objects from the contents of the files.
As the action method called by the Save All command, saves all open documents of the application that need to be saved.