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
WICKET-5467 NumberTextField should support "any" as valid step attribute value