tl_templates.php experiments (@see https://github.com/contao/contao/issues/1399)

tl_templates.php

<?php

$GLOBALS['TL_DCA']['tl_templates'] = array
(
	// Config
	'config' => array
	(
		'dataContainer'               => 'Folder',
		'validFileTypes'              => 'html5,twig', // add twig


// [...]


  public function adjustSettings()
	{
		Contao\Config::set('uploadPath', 'templates');
		Contao\Config::set('editableFiles', 'html5,twig'); // add twig
	}

  public function addNewTemplate()
  {
    $arrAllTemplates = array();

    /** @var Symfony\Component\Finder\Finder $files */
    $files = Contao\System::getContainer()->get('contao.resource_finder')->findIn('templates')->files()->name('/\.html5$/');

    // <new code>
    // Nur richtig unter der Annahme: "wo es ein Unterverzeichnis contao/ gibt, gibt es auch views/"
    // Works on my mashine ;-)
    $twigResourcesPaths = array_filter(
            array_map(
                function($path) { return preg_replace("/contao$/", 'views', $path); },
                Contao\System::getContainer()->getParameter('contao.resources_paths')
            ),
            function($path) { return is_dir($path); }
    );

    /** @var Symfony\Component\Finder\Finder $twigFiles */
    $twigFiles = \Symfony\Component\Finder\Finder::create()->in($twigResourcesPaths)->files()->name('/\.twig$/');

    $files->append($twigFiles);
    // </new code>