Two endpoints rather than a box: no text, no fill, and the diagonal of
the frame with `flipV` picking which one. The three kinds share their
preset names, since the heads a file declares are what tell them apart.
ODF gives a line its own element and its heads a named marker, which
needs the draw: prefix declared in styles.xml — undeclared, the file
does not parse at all. Word keeps the frame and flips it.
