[POC] implemented basic logic for cascading referrers removal

This is still missing a lot of things, it's not even checking if the referrers of a field should cascade remove at all.
I tried adding some of the needed checks but got stuck right away. Fetching the 'stored' items by using the DocumentManager's getReferrers function will result in a single collection with all the referrers linked to the document and there is no easy way to determine which referrer belongs to which field.
So I decided to just include the absolute minimum for this to work.

linked to https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/issues/142
cc @lsmith77