removeParent() performs very poorly when a parent has >10000 items

Children lists currently use QLists which perform terribly when the parent has many children - O instead of O(1).

Fix: ChildrenList should be made std::set instead of std::list, and the respective insert operation should be fixed. Affects files libshiboken/basewrapper.cpp, libshiboken/basewrapper_p.h.