static once(el, event, callback) {
    const cb = e => {
      if (e.target === el) {
        el.removeEventListener(event, cb);
        callback(e);
      }
    };

    el.addEventListener(event, cb, false);
  }
Listens for an event once.
@param {Element} el Element to listen to.
@param {string} event Event Type.
@param {Function} callback Function to execute once the event fires.