MenuItem.setSubmenu
void setSubmenu(gio.menu_model.MenuModel submenu = null)Sets or unsets the "submenu" link of menu_item to submenu.
If submenu is non-null, it is linked to. If it is null then the link is unset.
The effect of having one menu appear as a submenu of another is exactly as it sounds.
Parameters
submenu | a #GMenuModel, or null |