[POC] Use the Section VO instead of a custom entity

> Proof of concept

Feeds the Section form with the Section Value Object instead of using a custom object.

Requirements aren't respected, but the existing @require annotations could be replaced with the ones from Form. A next step would be to have 2 distinct forms for Create and Update,
so that they can have their own ValueObject.

Tested manually from `/pjax/section/create`. It works :-)

### Open questions

### Approach
@dpobel told me that this is the approach that was tried a couple month back, and that we didn't go for it for "reasons", one of them being form validation (see below).

#### Form validation
The most common way to validate forms is through

#### Existing contributions
- netgen/NetgenEzFormsBundle, written by @pspanja. Has a data wrapper for Content & User, and create/update forms for it. Uses a registry to convert fields.
- heliopsis/ezforms-bundle, written by @bchoquet-heliopsis. More mature than Netgen's solution.