Sets whether the view controller shows an editable view.
If YES, the view controller should display an editable view; otherwise, NO.
If YES and one of the custom views of the navigationItem property is set to the value returned by the editButtonItem method, the associated navigation controller displays a Done button; otherwise, an Edit button.
If YES, animates the transition; otherwise, does not.
Subclasses that use an edit-done button must override this method to change their view to an editable state if editing is YES and a non-editable state if it is NO. This method should invoke super’s implementation before updating its view.