using PropertyAccessor to work with entity properties (correct way for guess add/remove methods names, see: Symfony\Component\PropertyAccess\StringUtil::singularify())
example: entity has property "categories" and must have method addCategory (not addCategories)
Merge branch 'second_level_collection_add_button' of github.com:dypa/SonataAdminBundle into second_level_collection_add_button