Ensure child items are updated even if the parent item has no content

Ensure child items are updated even if the parent item has no content

When a parent item had its visiblity toggled, then the child item would
not update if the parent item had ItemHasNoContents and
ItemClipsChildrenToShape set. This is a common use case in declarative as
the root item has ItemHasNoContents set.

Task-number: QTBUG-26846

Change-Id: Id6592ebc4ba2caa4331a4a71f7247e40993131b6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>