Add a new `p:video` child element for movie having *shape_id*.

def add_video(self, shape_id):
        """Add a new `p:video` child element for movie having *shape_id*."""
        video_xml = (
            '<p:video %s>\n'
            '  <p:cMediaNode vol="80000">\n'
            '    <p:cTn id="%d" fill="hold" display="0">\n'
            '      <p:stCondLst>\n'
            '        <p:cond delay="indefinite"/>\n'
            '      </p:stCondLst>\n'
            '    </p:cTn>\n'
            '    <p:tgtEl>\n'
            '      <p:spTgt spid="%d"/>\n'
            '    </p:tgtEl>\n'
            '  </p:cMediaNode>\n'
            '</p:video>\n' % (nsdecls('p'), self._next_cTn_id, shape_id)
        )
        video = parse_xml(video_xml)
        self.append(video)