Support <buttons> inside <vob>

[lu-zero]

dvdunauthor generates a structure like

        <vob file="vob_01m_043.vob">
          <buttons start="0:00:00.000">
            <button name="1">
                jump pgc 33;
              </button>
            <button name="2">
                jump program 1;
              </button>
            <button name="3">
                jump program 2;
              </button>
            <button name="4">
                jump program 3;
              </button>
            <button name="5">
                jump program 3;
              </button>
          </buttons>
          <cell start="0:00:00.000" program="1" pause="inf"/>
        </vob>

And would be nice supporting it in dvdauthor.