'Replace the child ``oldchild`` with ``newchild``.'

def replace_child(self, oldchild, newchild):

    index = self._subtrees.index(oldchild)
    self._subtrees[index] = newchild
    self._remove_child_widget(oldchild)
    self._add_child_widget(newchild)
    self.update(newchild)
