Form type : sonata_choice_field_mask

Copied from SonataDoctrinePhpcrAdminBundle
This widget allow to hide/display other fields in a form when a value in a select widget  is selected.

This PR cancel and replace my previous one : https://github.com/sonata-project/SonataDoctrineORMAdminBundle/pull/373
Like proposed by pulzarraider I put it directly in SonataAdmin instead of  keep it in SonataDoctrineORMAdmin.