Add row selector for sonata_type_model_list

When opening a list from a ``sonata_type_model_list`` form field, this automatically add a "Choose" button for each row in the first column.

Choosing a row by clicking a link still works, but I think this way is less confusing.

![sonata_admin_choose](https://f.cloud.github.com/assets/663607/643819/313a97f8-d37e-11e2-8462-5e5d92e927c2.png)