/**
  * LazyestGallery::_subfolders()
  * Adds subfolders of $folder to the $folderlist
  * 
  * @param array $folderlist
  * @param LazyestFolder $folder
  * @param bool $show include hidden folders or not
  * @return
  */
 function _subfolders($folderlist, $folder, $show)
 {
     $subfolders = $folder->subfolders();
     if (0 < count($subfolders)) {
         foreach ($subfolders as $subfolder) {
             if ('hidden' == $show && 'hidden' == $subfolder->visibility || 'visible' == $subfolder->visibility) {
                 $folderlist[] = $subfolder;
                 $folderlist = $this->_subfolders($folderlist, $subfolder, $show);
             }
         }
     }
     return $folderlist;
 }