Function wxd_Menu_AppendSubMenu 

pub unsafe extern "C" fn wxd_Menu_AppendSubMenu(
    menu: *mut wxd_Menu_t,
    submenu: *mut wxd_Menu_t,
    title: *const c_char,
    helpString: *const c_char,
) -> *const wxd_MenuItem_t

@brief Append a submenu to a wxMenu. wxMenu takes ownership of the submenu.