added xfer<...> transfer() method to various containers

- this should provide a slightly more naturally means to using transfer
  constructors, for example
          labelList list2(list1.transfer());
      vs. labelList list2(xferMove(list1));

- returns a plain list where appropriate (eg, DynamicList, SortableList)
  for example
          labelList list2(dynList1.transfer());
      vs. labelList list2(xferMoveTo<labelList>(dynList1));