According to: http://www.opengroup.org/desktop/motif.html
Motif 2.x provides a
    *  Container/Icon Gadget that is used to provide both tabular and
       iconic presentations of a hierarchy of objects that can be used
       to build file/directory managers

This seems like something that could be done with the GtkTreeModel
and GtkCellRenderer. I think I read somewhere that something like
this is planned, but I didn't find anything related in bugzilla or
on the mailing lists.

This would seem a natural part of a toolbar customization mode
as mentioned in report #74329.