Add QGraphicsItem::childrenShape()

QGraphicsItem has a function childrenBoundingRect() which returns the boundingRect() of a parent item's descendants, transformed by the descendants' transform() and mapped to the parent item's coordinate system.

It would be nice if Qt had a corresponding function for shape(), childrenShape() for example, which returns the shape() of the parent item's descendants, transformed by the descendant's transform() and mapped to the parent item's coordinate system.