refactor Item to interface

[Joe Fawzy]

Hi allcan we refactor org.apache.wicket.markup.repeater.Item to an interface
IItem or DataItem or so , and make the standard Item class implement this
interface , this will maintain backward compatibility but allow us to use
panels and fragments as Item implementation (by overriding the newItem()
method) instead of being restricted to MarkupContainer which is not as
reusable as panels and fragments
thanks
joe