Allow PointerHandlers to set priority: receive events either before or after the handler's parent item

We aren't sure if this is needed, but it has been brought up that we don't yet have equivalent functionality as the Keys.priority property (can set it to Keys.BeforeItem or Keys.AfterItem).

It seems that if we want to provide it, we will have to iterate each item's pointer handlers in additional places in QQuickWindow, before and after delivery to the item, and deliver only to handlers which have the right priority set.

This may turn out to be most useful for the Pointer attached property.