[BUGFIX] File Relations don't work with workspaces

The file repository used to resolve all sys_file_references
handles enable fields manually currently, which is bad because
it forgets to handle the workspace-related where clause.

The attached patch splits the behavior into frontend and
backend context. In backend the RelationHandler is capable
of automatically resolving IRRE structures. In frontend mode
the enableFiels and version preview settings are relevant to
show correct results.

Resolves: #43916
Releases: 6.2
Change-Id: I6c19e5d7b4cd7a2134921c669b82ab90b54844de
Reviewed-on: https://review.typo3.org/29712
Tested-by: Lorenz Ulrich
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter