Option to show hierarchy in "read-only" mode

I've rerolled the patch against the latest release (2.1.4), and updated the issue summary with a (hopefully) clearer description.

FWIW, I'm not sure the solution in the patch (ie. adding a "read-only" option) is the best approach here. For example, this limits us to using a "table" format.

Instead, we might want to consider storing a "depth" value (similar to "parent"). This should allow us to reflect hierarchy in list formats too.