QQmlInstantiator leaks memory

Run the declarativeAssignViaAttached() test in tst_qquickvisualdatamodel.cpp and observe the leakage. This is because QQmlInstantiator insists on re-parenting the objects it receives from QQmlDelegateModel. QQmlDelegateModel, however, keeps some metadata on those objects and wants to release it when releasing the objects in its dtor. In particular scriptRef and contextData. Furthermore, QQmlInstantiator should run QQmlInstantiatorPrivate::clear() in its dtor so that any QQmlDelegateModel that may be supplying the objects knows it doesn't need them anymore.