[Vala] Gtk.TreeView expander on the right

[david <davidrafagomes gmail com>]

http://valadoc.org/gtk+-3.0/Gtk.TreeView.html

There's some information on columns and the "expander_column" on that 
page, but I can't find a way of setting the expander to appear on the 
right of the *last visible column* of the Tree View, because I don't 
know how to get the last visible column.

How can I get that column? Thanks.