This commit adds the basics to bridge the blink
accessibility classes to QAccessibleInterfaces.

Note that it needs two follow up commits to implement the bridging from
the QWidget/Qt Quick worlds.

[ChangeLog][Accessibility] QtWebEngine now has accessibility
support, enabling assistive technology such as screen readers to work
with it.

Change-Id: Ied1d97e61a024115ac7a9245331211f6d9fac1b4
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>