An ElementType should answer "true" to isSimple if it can be edited in a grid.

But ElementType.isSimple is used by SitemapSaxer and AmetysObjectElement to kwow if the value can be saxed into a XML attribute, and so do a toString on the value.

The method isSimple is used here for another semantic. It is not about to edit the value in a grid here but to represent the value as a String.

Maybe a ElementType should have another method named isStringify? isStringable? which should be used by SitemapSaxer or AmetysObjectElement.

Or we still use isSimple in those classes but all simple types should implement the toString method