Generic Undo/Redo:
I took a look into the web and found the gundo project ("A Multilevel Undo/Redo Facility for Gtk"); I ported this from GTK+ 1.2 to GTK+ 2.0 and now I'm starting to make it look the way I would want to use it in an application:
* have an Interface that can be implemented by applications document classes
* have some GtkToolItems for undo/redo that can be used for undo/redo widgets in toolbars
* have custom GtkAction elements for easy embedding into the application, etc
GUndo still only lives on my computer as I don't think that my changes are yet as clean as I want them to be. To be published soon...
