Sets the selection mode for the specified CList. This allows you to
set whether only one or more than one item can be selected at a time
in the widget. Note that setting the widget's selection mode to one
of GTK_SELECTION_BROWSE or GTK_SELECTION_SINGLE will cause all the
items in the GtkCList to become deselected.
WARNING: gtk_clist_set_selection_mode is deprecated and should not be used in newly-written code.