Impossible to determine if an QML element is above another

To be able to determine which of two (random) QML elements are on top
in z-order screen space, some new API is needed. E.g. a
bool Item::isAbove(Item)
or
int Item::absoluteZ()
The former seems easier to use, while the latter may be more efficient for
e.g. sorting many elements. But I suspect the latter cannot be implemented
due to the z-positions relativistic nature.