Change data-type url to urlstrict for urlvalidator

If value 'www.wicket.org' is entered the clientside validation works but on the serverside a strict check is performed (scheme is mandatory) and this causes the serverside validation to fail but on the clientside you don't see anything.

... thinking about this: If this was intended behaviour this should be documented somewhere ;-)

Signed-off-by: Andreas Kuhtz <andreas.kuhtz@gmail.com>