_getFilteredSets(category, items) {
    let visible = [];
    const hidden = [];

    // category === 'all', add visible class to everything
    if (category === Shuffle.ALL_ITEMS) {
      visible = items;

      // Loop through each item and use provided function to determine
      // whether to hide it or not.
    } else {
      items.forEach((item) => {
        if (this._doesPassFilter(category, item.element)) {
          visible.push(item);
        } else {
          hidden.push(item);
        }
      });
    }

    return {
      visible,
      hidden,
    };
  }
Returns an object containing the visible and hidden elements.
@param {string|string[]|function(Element):boolean} category Category or function to filter by.
@param {ShuffleItem[]} items A collection of items to filter.
@return {{visible: ShuffleItem[], hidden: ShuffleItem[]}}
@private