Return newly-appended textbox `p:sp` element.

        Element has position (*x*, *y*) and size (*cx*, *cy*).

def _add_textbox_sp(self, x, y, cx, cy):
        """Return newly-appended textbox `p:sp` element.

        Element has position (*x*, *y*) and size (*cx*, *cy*).
        """
        id_ = self._next_shape_id
        name = 'TextBox %d' % (id_-1)
        sp = self._spTree.add_textbox(id_, name, x, y, cx, cy)
        return sp