Allow QSGItem to notify about absolute position changes and value

Add to QSGItem an equivalent to QGraphicsItem::ItemSendsScenePositionChanges and QGraphicsItem::mapToItem(). This would allow other items to get notified on changes of the absolute positions of a given item and get the value of that absolute position.

This is really useful when one wants to draw elements that connect QSGItem s that don't share the same parent (e.g. a connecting line or curve between two items of different list views).