onClick_(event) {
    if (CLICKABLE_ELS.indexOf(event.target.tagName) < 0 &&
      typeof event.target.dataset.preventNav === 'undefined') {
      this.ws_.goNext();
    }
  }
Reacts to the click event. It will go to the next slide unless the element
has a data-prevent-nav attribute or is on the list of CLICKABLE_ELS.
@param {MouseEvent} event The click event.
@private