[WIP] filter children phpcr-odm tree

See https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/84

Started by using the already existing config, it was a try but seems to work pretty nice. Do we then still have a need to hide children based on a pattern?

Todo:
- filter the form types, for instance to only show pages when a page needs to be selected