forward declarations of gtkmm stuff

["David L" <idht4n gmail com>]

Sorry if this is a stupid question, but how can I forward declare 
gtkmm things so that gtkmm header files don't need to be 
included before defining a class that has gtkmm pointers?

For example, I'd like to get rid of the two includes in this header file:
// foo.h
#include <gtkmm/window.h>
#include <gtkmm/radioaction.h>
class foo {
 public:
  foo();
 private:
  Gtk::Window *myWindow;
  Glib::RefPtr<Gtk::RadioAction> myRadioAction;
};

Thanks,

                   David