constructor(el, i) {
    /**
     * @type {Element}
     */
    this.el = el;
    /**
     * The section's parent.
     * @type {Node}
     */
    this.parent = el.parentNode;
    /**
     * @type {number}
     */
    this.i = i;

    this.el.id = this.el.id ? this.el.id : `section-${(i + 1)}`;
    this.el.classList.add(CLASSES.SLIDE);

    // Hide slides by default
    this.hide();
  }
Bootstraps the slide by saving some data, adding a class and hiding it.
@param {Element} el Section element.
@param {number} i Zero based index of the slide.