First cut at adding a comment field to listings.

The jury is still out on some details...
1) If you press ESC while editing the comment, it currently cancels
   the edit.  Would people expect ESC to exit the program instead?
2) The format/colours of the displayed comment... right now it's
   using the UIFC background colours, and not draing a border or
   window.
3) Just how long should the comment field be, and how should it be
   edited.