add RequestClose web action

Web pages can set the onbeforeunload handler to let the user confirm
whether to leave the page or not. Until now, only when leaving the
page via a link, a confirmation was shown.
Before actually closing a web page, applications can now trigger
the RequestClose web action. This will give the use the chance to
confirm or deny the close request.
If the request is confirmed, the signal windowCloseRequested is
emitted.

Task-number: QTBUG-36155
Change-Id: Icc1fabc37a2ac537f674c2f00bc8966e4dc4e610
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>