Reduced the coupling with reflection objects

For now, the main benefit is that Doctrine/Propel entities are no longer
required to generate file links (with Twig `vich_uploader_asset`
function for instance).

The following becomes possible:

```html+jinja
{{ vich_uploader_asset(entry, 'imageFile', 'FooBundle\Entity\Entry') }}
```

Where `entry` isn't an object but an array.

This would close the issue #138