Append a newly-created placeholder ``<p:sp>`` shape having the
        specified placeholder properties.

def add_placeholder(self, id_, name, ph_type, orient, sz, idx):
        """
        Append a newly-created placeholder ``<p:sp>`` shape having the
        specified placeholder properties.
        """
        sp = CT_Shape.new_placeholder_sp(
            id_, name, ph_type, orient, sz, idx
        )
        self.insert_element_before(sp, 'p:extLst')
        return sp