WICKET-5071 404 Error on Nested ModalWindows in IE7 and IE8

Extend PageInstanceMapper to match when the request is to 'page?...' and the baseUrl is 'wicket/page?...'
WICKET-5071 404 Error on Nested ModalWindows in IE7 and IE8

Make the check more strict. We can relax it later if needed.
WICKET-5179 Move Form hidden field rendering to dedicated method