.h:
class ItemNotes : public WithItemNotes<TopWindow> {
	typedef ItemNotes CLASSNAME;

        Download_Manager_Window &download_manager_window;
...
};

.cpp:
	ItemNotes::ItemNotes(int item_id,Download_Manager_Window &_download_manager_window) : download_manager_window(_download_manager_window)
{
// Constructor
}