I just realize adding a querybuilder or closure as setting is not a good idea:
* see https://github.com/symfony-cmf/BlockBundle/issues/21 -> it is better for settings to be persisted to be able to cache them properly
* a querybuilder or closure cannot be persisted in a phpcr multivalue string
* and will it be useable? how to pass a querybuilder or closure from ''sonata_block_render''?