Add row selector for sonata_type_model_list

When opening a list from a ``sonata_type_model_list 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/663607/643575/7dd85204-d378-11e2-9d61-fe29379612d7.png)