[WIP][2.4][Form] Add option "widget" to ChoiceType

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | no
| Fixed tickets | #6602
| License       | MIT
| Doc PR        | not yet

- [ ] add / fix tests
- [ ] submit changes to the documentation
- [ ] update php templates and twig table layout

I've implemented the behavior described in #6602.

### Choice

- About the ```text``` widget, I think it makes more sense to ask for the label instead of the value, 

- Also, does ```choice_widget_select``` now fit better than ```choice_widget_collapsed```?

### Doctrine

- For the original entity_id use case, it'd simply mean setting the ```property``` option to ```id```.

``` php
<?php

$builder->add('field', 'entity', array(
    'class' => 'MyBundle:Product',
    'widget' => 'text',
    'property' => 'id',
));
```

Then the choice list won't be loaded in advance but validated on the fly.

I had to add 2 methods in the choiceList for this feature (getChoicesForLabels and the reversed one). What do you think about the approach?  cc @bschussek