[gtkada] Set_Column_Visibility

[manuel.op.de.coul at eon-benelux.com]

In GtkAda 1.2 there's a mysterious comment in the Gtk.Clist spec for
Set_Column_Visibility:

-- Note that GtkAda prevents the last visible column to be hidden.

But I had no problem doing that, you only need to remember
to start counting from 0.

Manuel