Added custom labels and possibility to remove button sonata_type form types

On 'sonata_type' form fields, add the possibility of changing the "add new", "list" and "delete" buttons labels, when they are present. If set to false, the button is not displayed. Defaults were added so that no BC ocurrs. Docs updated. 

These changes are also affect SonataAdmin, so the corresponding PR will also need to be merged on it in too. After these changes have been reviewed, I'll copy them to SonataDoctrineMongoDBAdminBundle.