relation-discriminator extension should work with N-1 relationships

[andyjefferson]

relation-discriminator-column should apply to n-1 join tables. Currently, only ElementContainerTable deals with it, which means that the parent must have a Collection of children for it to work. Since N-1 is a subset of N-M, I think it should be fairly trivial to implement this issue

When asked to implement it due to it "being trivial", the response was along the lines of "oh, I can't do things like that". Raised on the old DN forum.