Expose QQuickItemPrivate::paintOrderChildItems (or some other way to access items in visual order)

When implementing a variety of things like hit testing, or customized/tweaked event delivery, it is almost always necessary to make use of QQuickItemPrivate::paintOrderChildItems to do so (as QQuickWindow itself does for event delivery).

Similarly, the usual loop over it (checking isVisible / isEnabled / isCulled) would be nice to have in some sort of a reusable form. Maybe the right thing to do with this is to turn it into an iterator, and make that public.

This would go a very very long way to stamping out use of private API that I've written (or seen) around the place.