GtkCTree changes

[Lars Hamann <lars gtk org>]

Hi!
 
I've committed a patch for GtkCTree to use "GtkCTreeNode * node" instead of
"GList * node" to increases type safety. 
 
GtkCTreeNode is defined according to Owens proposal :
 
struct _GtkCTreeNode {
  GList list;
};
 

bye,
  Lars