Allow changing Product Hierarchy assignments of parts when the parts themselves doesn't belong to the current portion of a split project, but the parts' Unit does.
For example, consider the following situation:
1. The user is working in a split project.
2. The project is split into two portions: Split A and Split B.
3. As a whole, Unit X is controled by Split A; however, some individual part types inside Unit X are delegated to Split B.
If the user attempts to move parts that are assigned to Split B in the Product Hierarchy tree of Split A, he or she won't be able to do it. In the correct situation, if a Unit belong to a split portion but some parts don't, it should be allowed to move parts in the PH tree inside the owned Unit; however, after the move is done, parts' names should remain unchanged.
