[gollem] backend configurable header displays

[Liam Hoekenga]

> Committed. I understand it might not be perfect yet, but we'll sort it out.

The preference gets displayed as it should in the options screen.. and lets you
add and remove columns, and re-order them, but it doesn't actually seem to save
that preference when you "save options".  It's lifted almost verbatum from
Turba, so maybe I forgot something.

The preferences screen also is only displaying the "internal" names for those
columns (eg "name" instead of "File Name", "modified" instead of "File
Modification Date", etc).  Depending on how far the file system attributes get
abstracted, it might make sense to move more towards Turba's "sources" / "map"
and "attributes" model.  I don't know, tho.. I haven't done a lot of application
design.

Liam