(_widget_value): Added lname and lkey.

(_widget_value): Added lname and lkey.
(digest_single_submenu): Set lname and lkey in widget_value
instead of name and key.
(update_submenu_strings): New function.
(set_frame_menubar): Remove call to inhibit_garbage_collection,
call update_submenu_strings.

(globals_of_w32menu): Check for Unicode API.
(digest_single_submenu, w32_menu_show): Encode menu strings as
UTF-8 if Unicode API is available.
(utf8to16): New function.
(add_menu_item): Use it when calling Unicode API.