(enum menu_item_idx): New enumeration replacing defines

(enum menu_item_idx): New enumeration replacing defines
MENU_ITEMS_ITEM_.*.
(MENU_ITEMS_ITEM_HELP): New enumerator.
(push_menu_item): Add parameter HELP.  Record help in menu_items.
(single_menu_item, single_submenu, list_of_items): Call
push_menu_item with new parameter.
(single_submenu): Set help string in widget value.
(menu_highlight_callback): New function.
(set_frame_menubar): Call lw_create_widget with new
parameter list.
(xmenu_show, xdialog_show): Ditto.