Use PropertyAccess for extend usability of getFieldValue

Current implementation is a bit too light (getter/isser) when PropertyAccess already used elsewhere in this bundle offer more (hasser/magic function)
This allow bundles like A2lixI18nDoctrine Bundle and KnpLabsDoctrineBehaviors which allow a __call method as proxy method to work with ease with Sonata.

See http://a2lix.fr/blog/2014/08/04/translate-internationalize-doctrine-entities-sonata.html

Any thoughts?
No it can't. https://github.com/symfony/PropertyAccess/blob/master/PropertyAccessor.php#L52
But with comment ```` "// parameters for the method given in the code option" ```` https://github.com/sonata-project/SonataAdminBundle/pull/2262/files#diff-67a6ff31e9d77cbbe3c75d3e24734453L337, I supposed that was only used with code option and so, do not manage this case with PropertyAccess.