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>