code: protected function getTargetFolders($strFolder, $intLevel=1)
  	{
  		$strFolders = '';
  		$strPath = Contao\System::getContainer()->getParameter('kernel.project_dir') .'/'. $strFolder;
  
  		foreach (scan($strPath) as $strFile)
  		{
  			if (!is_dir($strPath .'/'. $strFile) || strncmp($strFile, '.', 1) === 0)
  			{
  				continue;
  			}
  
  			$strRelPath = $strFolder .'/'. $strFile;
  			$strFolders .= sprintf('<option value="%s"%s>%s%s</option>', $strRelPath, ((Contao\Input::post('target') == $strRelPath) ? ' selected="selected"' : ''), str_repeat(' &nbsp; ', $intLevel), basename($strRelPath));
  			$strFolders .= $this->getTargetFolders($strRelPath, ($intLevel + 1));
  		}
  
  		return $strFolders;
  	}
docs: Recursively scan the templates directory and return all folders as array
  
  @param string  $strFolder
  @param integer $intLevel
  
  @return string