[TASK] Refactor file references relations retrieval

The retrieval of file references in BackendUtility
is currently bound to the thumbCode() method but
is useful for other components as well.

Resolves: #60054
Releases: master, 6.2
Change-Id: Id92bdeaed24d8924285a640f905ffd55db21da4d
Reviewed-on: http://review.typo3.org/31287
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>