function addMarkerButtons (buttons) {
      Object.keys(markerTypes).forEach(function (id) {
        const title = getTitle(String(id));
        buttons.push({
          id: 'placemark_marker_' + id,
          svgicon: id,
          icon: 'markers-' + id + '.png',
          title,
          type: 'context',
          events: { click: setArrowFromButton },
          panel: 'placemark_panel',
          list: 'placemark_marker',
          isDefault: id === 'leftarrow'
        });
      });
      return buttons;
    }
Build the toolbar button array from the marker definitions.
@param {module:SVGEditor.Button[]} buttons
@returns {module:SVGEditor.Button[]}