Support more widgets in DC_Folder

[fritzmg]

Currently it is not possible to use a widget within DC_Folder that executes an ajax request - e.g. fileTree. So something like this does not work:

// contao/dca/tl_files.php
use Contao\CoreBundle\DataContainer\PaletteManipulator;

$GLOBALS['TL_DCA']['tl_files']['fields']['alternative'] = [
    'label' => ['Alternative file', 'Alternative file reference.'],
    'inputType' => 'fileTree',
    'eval' => [
        'extensions' => Contao\Config::get('validImageTypes'),
        'filesOnly' => true, 
        'fieldType' => 'radio', 
        'tl_class' => 'clr',
    ],
    'sql' => 'binary(16) NULL',
];

PaletteManipulator::create()
    ->addField('alternative', 'meta')
    ->applyToPalette('default', 'tl_files')
;

If you select a file via this file picker, the resulting ajax request will fail:

Symfony\Component\HttpKernel\Exception\BadRequestHttpException:
Bad request

  at vendor\contao\contao\core-bundle\src\Resources\contao\classes\Ajax.php:318

System log entry:

A record with the ID "files/lorem/ipsum.jpg" does not exist in table "tl_files"

This is because the "ID" within DC_Folder will always be the relative file path, i.e. tl_files.path and not the database ID, i.e. tl_files.id.

This PR would fix that by automatically switching to tl_files.path for the database query, if the datacontainer is an instance of DC_Folder.

[Toflar]

I don't think this is the right place to do that but also it's a quick fix and there's a lot of specifics here already so it doesn't hurt.

[leofeyer]

Thank you @fritzmg.