function colorChanged (el) {
      const color = el.getAttribute('stroke');
      const marker = getLinked(el, 'marker-start');
      if (!marker) { return; }
      if (!marker.attributes.class) { return; } // not created by this extension
      const ch = marker.lastElementChild;
      if (!ch) { return; }
      const curfill = ch.getAttribute('fill');
      const curstroke = ch.getAttribute('stroke');
      if (curfill && curfill !== 'none') { ch.setAttribute('fill', color); }
      if (curstroke && curstroke !== 'none') { ch.setAttribute('stroke', color); }
    }
Called when the main system modifies an object. This routine changes
  the associated markers to be the same color.
@param {Element} el
@returns {void}