Add NonResettingRestartException

In my application, I use a NonResettingRestartException as suggested by Martin Grigorov in the mailing lists. This exception triggers a redirect which restarts the response but does not discard the response's meta data (e.g. cookies). Its implementation and discussion are in WICKET-4637.

In my opinion, such an "official" version of this exception would be useful in wicket-core since it's not at all trivial to come up with unless much of Wicket internals is known.