Existing Menu/Toolbar API

Code for creating menus looks a bit like this:
Create GtkMenuBar
Add GtkMenuItems for toplevel menus, and attach GtkMenus to them.
Add GtkMenuItems to the GtkMenus
Attach callbacks to the "activate" signal of the menu items
Repeat for sub-menus ...
There is GtkItemFactory to simplify things, but it does essentially the same thing as this
Toolbars are similar, although no sub-menus.