updateCounter(current, max) {
    if (this.ws_.options.showIndex) {
      this.counter.childNodes[0].textContent = `${current} / ${max}`;
    } else {
      this.counter.textContent = `${current} / ${max}`;
    }
  }
Updates the counter inside the navigation.
@param {string|number} current Current slide number.
@param {string|number} max Max slide number.