Add a way to detect form resubmissions on POST reloads

QtWebKit had NavigationTypeFormResubmitted to detect when a form gets resubmitted via e.g. a reload (test page: http://httpbin.org/forms/post). Firefox and Chromium warn/ask the user as well, as this could lead to e.g. unintended double purchases.
Nothing equivalent seems to exist in QtWebEngine, unless I'm missing something.