Append a new freeform `p:sp` with specified position and size.

def add_freeform_sp(self, x, y, cx, cy):
        """Append a new freeform `p:sp` with specified position and size."""
        shape_id = self._next_shape_id
        name = 'Freeform %d' % (shape_id-1,)
        sp = CT_Shape.new_freeform_sp(shape_id, name, x, y, cx, cy)
        self.insert_element_before(sp, 'p:extLst')
        return sp