file directory and message sub-board lists for ones with common groups, adding a 3rd
tier of options.  For example, if you have the following file directory structure:
BBS files
  DOS: BBS software
  DOS: BBS doors
  DOS: BBS utilities
  Windows: BBS software
  Windows: BBS doors
  Windows: BBS utilities

With directory name collapsing, the selection will be come 3 tiers, as follows:
BBS files
  DOS
    BBS software
    BBS doors
    BBS utilities
  Windows
    BBS software
    BBS doors
    BBS utilities