Replace accessibility private API usage with QQmlProperty

Previously enabling accessibility for the QtQuick WebEngineView was
done by calling QuickAccessibleAttached::qmlAttachedProperties()
which is a private API call.
Instead of that, set the Accessibility attached properties by using
QQmlProperty.

Task-number: QTBUG-63098
Change-Id: Ibc927c1fce121dee56d8a21af412056d98c82c4d
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>