Extend fileTree Widget to configure gallery and download type by eval

To reproduce:
1. create a custom Module
2. Create a DCA
3. Add more than one `fileTree` Widget to one Module Form
4. It is not possible to define for each `fileTree` Widget `gallery` or `download`. It is always for the whole Form / Type

This pull request allows developers to have some fileTree widgets in one Backend Form and define one as `download` the other one as `gallery` and so on.

Example:
```php
// gallery
'eval'  => array('gallery'=>true),
// download
'eval'  => array('download'=>true),
```

```php
'multiSRC' => array
(
	'label'                   => &$GLOBALS['TL_LANG']['tl_content']['multiSRC'],
	'exclude'                 => true,
	'inputType'               => 'fileTree',
	'eval'                    => array('multiple'=>true, 'fieldType'=>'checkbox', 'orderField'=>'orderSRC', 'files'=>true, 'mandatory'=>true, 'gallery'=>true),
	'sql'                     => "blob NULL"
),
```