[BUGFIX] Workspace not always applied in RelationHandler queries

The possibly submitted value to RelationHandler::setWorkspaceId()
is not used for SQL queries to read IRRE foreign field relations.
In that case the current $BE_USER->workspace value is used which
might be different to the requested workspace id.

Resolves: #60053
Releases: 6.2, 6.3
Change-Id: Ib01e4df798ff2e1fcc8c29997f6effb814dc4344
[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: 6.2, 6.3
Change-Id: Id92bdeaed24d8924285a640f905ffd55db21da4d
[WIP][TASK] Missing visual representation of sys_file_reference

Resolves: #60011
Change-Id: I6d22619c264ff0e5411a47b2d566ec2c9b7c2607