ListIterator: Introduce an overload accepting a QList<T> getter

Introduce an overload taking a function returning QList<T>.
The function is going to be called just once by the running task tree
when the Group containing the iterator is entered.
All relevant storages and parent iterators are activated during
the call.

This is going to be useful when iterating over nested loops.
The inner loop may conveniently iterate over outer's current value.
See NestedLazyList test.

Change-Id: I7ad9602ac5f01a93ecd835187593ee3d03665660
Reviewed-by: Marc Mutz <marc.mutz@qt.io>