Some time during the 1.3.x development cycle, the GtkCTreeNode arguments to
the GtkCTree signals were changed from GTK_TYPE_CTREE_NODE's to
GTK_TYPE_POINTER's.  I believe this was done because the CTreeNodes don't
follow the standard boxed memory management model.

This change doesn't really affect C programmers, however it makes those
signals useless in language bindings which use the GType type information
to decide how to wrap a signal argument.  This means that programmers using
such a language binding would need to change over to GtkTree* straight away.

The following patch adds a new boxed type for GtkCTreeNodes using no-op
copy/free functions, to add type information to the signal arguments.  This
makes the widget usable for programmers using a LB while they are
transitioning programs to the new widgets.