Create accessible interfaces only for items with attached prop

Create accessible interfaces only for items with attached prop

If the attached property is not found, ignore the item, unless
it is part of the tree. We still create a valid hierarchy this
way, but filter out all irrelevant items. The same goes for
hit-testing.

On the other hand this patch enables the root item to be found
by setting isAccessible on it when updating the flags. Otherwise
the hierarchy is not valid again.

Change-Id: Ied422fd0506d13458757c87a5dad7cdb9d3079bf
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>