There is currently no way to allow a cell renderer to span multiple columns for
a single row.

There are many places where this is useful, for example, if you have a treeview
that has a concept of 'categories' as the root nodes, and data underneath,
currently you have to allow the first column to be as wide as the category text,
instead of being able to say 'for this row, the cell spans all 3 or 4 columns'

There are some ways around it, but they all involve subclassing treeview, and
drawing to the BinWindow.