FIX: Let GridFieldEditableColumns edit relations via dot syntax
FIX: Let GridFieldEditableColumns edit relations via dot syntax

This is a companion to https://github.com/silverstripe/silverstripe-framework/pull/9192 to provide the same functionality for inline
editing in GridFields

A valuable use of this is editing fields in the join-object of
a many-many-through relation.
symbiote/silverstripe-gridfieldextensions,symbiote/silverstripe-gridfieldextensions