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