WICKET-5469 ModalWindow unload warning always displayed (even if window is closed)

Use event listeners instead of property based event handler for onbeforeunload.

We don't do the same for 'onunload' because event listeners can be skipped with event.stopImmediatePropagation() and this may lead to memory leaks in IE.
WICKET-5468 UrlRenderer#renderRelativeUrl potentially appends "/" after query parameters

(cherry picked from commit d9f50272e37b62a78f6a67c9b9f3f32f024d0ab8)
WICKET-5467 NumberTextField should support "any" as valid step attribute value

(cherry picked from commit 2c3aa01aba8666f8dc358e5ae25f36e38523924d)