Widget based accessibility glue code

With this patch the accessibility hierarchy inside webengine becomes
available when navigating from QWidget based web views.

Change-Id: Ib3625a6ec93b4d3f298fb845ab85209b348349ba
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>