Add core accessibility

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>