Move an item down in the list.

        Essentially swap it with the item below it.

        :param item: The item to be moved.

def move_item_down(self, item):
        """Move an item down in the list.

        Essentially swap it with the item below it.

        :param item: The item to be moved.
        """
        next_iter = self._next_iter_for(item)
        if next_iter is not None:
            self.model.swap(self._iter_for(item), next_iter)