/**
  * Return "Add to Wishlist" button.
  * 
  * @since 1.0.0
  */
 static function add_to_wishlist($atts, $content = null)
 {
     global $product, $yith_wcwl;
     $html = YITH_WCWL_UI::add_to_wishlist_button($yith_wcwl->get_wishlist_url(), $product->product_type, $yith_wcwl->is_product_in_wishlist($product->id));
     $html .= YITH_WCWL_UI::popup_message();
     return $html;
 }