static getSectionFromEl(el) {
    let parent = el;
    let section = null;
    let i = null;

    while (parent.parentElement &&
          !parent.classList.contains(CLASSES.SLIDE)) {
      parent = parent.parentElement;
    }

    if (parent.classList.contains(CLASSES.SLIDE)) {
      section = parent;
      i = parseInt(section.id.replace('section-', ''), 10);
    }

    return {section, i};
  }
Gets the section element from an inner element.
@param {Node} el
@return {{section: ?Node, i: ?number}} A map with the section and the
position of the section.