Add child to children of this TreeItem

        :param child: the child TreeItem
        :type child: :class:`TreeItem`
        :returns: None
        :rtype: None
        :raises: None

def add_child(self, child):
        """Add child to children of this TreeItem

        :param child: the child TreeItem
        :type child: :class:`TreeItem`
        :returns: None
        :rtype: None
        :raises: None
        """
        child.set_model(self._model)
        if self._model:
            row = len(self.childItems)
            parentindex = self._model.index_of_item(self)
            self._model.insertRow(row, child, parentindex)
        else:
            self.childItems.append(child)