Add a WebEngine.experimental QtQuick plugin library.

Bring this QtWebKit concept to our API as well to contain APIs that
we aren't comfortable adding to the supported-forever group yet and
allow trying them out unofficially for an undetermined number of
releases first.

Change-Id: I52c8655dfd2996ea461ac6c00de975002827a4c3
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Arvid Nilsson <anilsson@blackberry.com>
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>