Validates the Revert menu item and items selected from the Save panel’s pop-up list of writable document types items.
an should be enabled,
NO otherwise. Returns
YES for Revert if the document has been edited and a file exists for the document. Returns
YES for an item representing a writable type if, during a Save or Save As operation, it is a native type for the document. Subclasses can override this method to perform additional validations.