onBodyClicked_(event) {
    const matches = document.body.matches || document.body.msMatchesSelector;
    let el;

    if (matches.call(event.target, '[data-slide]')) {
      el = event.target;
    } else if (matches.call(event.target, '[data-slide] *')) {
      el = event.target.querySelector('[data-slide]');
    }

    if (el) {
      event.preventDefault();
      const i = this.slides.indexOf(el.dataset.slide);
      this.ws_.goToSlide(i);
    }
  }
Whenever the body is clicked, check if the element has [data-slide] attr
and if so, navigate to it.
@param {MouseEvent} event Click event