addGraphButtons(buttonEvents) {

    const parent = utils.getFirstElementByClassName('vis-navigation', this.domNode);

    for (let name in buttonEvents) {
      const div = this.document.createElement('div');
      div.className = `vis-button tmap-${name}`;

      div.addEventListener('click', buttonEvents[name].bind(this), false);
      parent.appendChild(div);


      this.setGraphButtonEnabled(name, true);

    }
  }
This function will create the dom elements for all tiddlymap-vis
buttons and register the event listeners.

@param {Object<string, function>} buttonEvents - The label of the
    button that is used as css class and the click handler.