[POC][WIP] Validate a product through validator directly (and not through Form)

# TL;DR:

This PR allows to validate a product by doing:

``` php
$violations = $this->validator->validate($product);
```

Usage of product_edit_form to validate a product should not be necessary any longer.

# Usage

To avoid breaking the whole PIM, the validator which is able to validate a product is registered under the service id `pim_validator`, be sure to inject it if you want to validate a product.

# Refactoring

IMO, it's no longer necessary to inject the constraints guesser in all of our attribute types.
It's been used to generate a form option "constraints" which contained the constraints to apply on the value data. As this constraints are now part of the property metadata, they should be validated when submitting the form.