Create example for full screen feature in WebEngine

Adds an example ('videoplayer') showing how to enable the Fullscreen
API[1] in QWebEngineView. This is one of the missing examples blocking
the removal of demobrowser.

[1]: https://fullscreen.spec.whatwg.org



Task-number: QTBUG-59820
Change-Id: Ib02a1556515d87e595ca54c2bce18c9144030fbc
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>