Append a newly-created textbox ``<p:sp>`` shape having the specified
        position and size.

def add_textbox(self, id_, name, x, y, cx, cy):
        """
        Append a newly-created textbox ``<p:sp>`` shape having the specified
        position and size.
        """
        sp = CT_Shape.new_textbox_sp(id_, name, x, y, cx, cy)
        self.insert_element_before(sp, 'p:extLst')
        return sp