menu and item handling in menubar manipulations
inside dtkWidgetsMenu::insertMenu / dtkWidgetsMenu::insertItem / dtkWidgetsMenu::add*
two private lists are manipulated (menu_list, object_list). The management of these lists are not coherent for example in dtkWidgetsMenu::insertMenu, the position parameter (pos) is used with both lists which is a bug.
This prevent to mix items and menus in menubars or menus.