Add a way to set the QWebEngineView scroll position

Porting an application from Qt Webkit to Qt WebEngine. The web page displays application data, not HTTP content, and all navigation is handled internally, by overloading QWebEnginePage::acceptNavigationRequest().

Qt Webkit QWebPage had a way to get and restore the current scroll position through

somepos = mainFrame()->scrollPosition(),mainFrame()
mainFrame()->setScrollPosition(somepos)

QWebEngine page now (5.7) has a scrollposition property, but it is readonly as far as I can see from the documentation.

Being able to set the scroll position is important so that the page position can be restored when the user moves back in the history.