#18 Smalltalk browser-like interface rather than tree.

[David M. Cook]

As suggested by the HIG, an interface like the NeXT
file browser or the Smalltalk class browser might be
more usable than trees (no problem with losing sight of
the parent; easier to keep track of what column you're
in). It has additional advantages to the programmer:

* "Lazy" loading is trivial.
* Uses ListStores instead of TreeStores.

However, cacheing would have to be implemented "by
hand", or avoided altogether (which may be unacceptable
performance-wise).