Add javascript dialogs support

This is just the basic core part and widgets plumbing and default
implementations of the virtual functions in QWebEnginePage.

QtQuick implementation is still yet to be done

Change-Id: I7cf8d6e5ec0bf747d45e9914db57bd0e4ef95b7f
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>