gtktreeview widget extension

["Mark Kegel" <mark kegel gmail com>]

I was wondering if someone had written a wrapper widget around
GtkTreeView that would create another tree view next to the original
whenever an expander was clicked, instead of inserting the entries
into the rows beneath. You can think of this like how OS X's Finder
does column view, with new columns appearing whenever a directory is
"expanded". It seems like most of the functionality is already there.
If no one has, then I have some work to do. Thanks.

Mark Kegel