bug#68979: Outline mode menu

[Juri Linkov]

While adding new commands ‘outline-cycle’ and ‘outline-cycle-buffer’
to the outline mode menu, I noticed a strange menu definition.

It looks like "Hide", "Show" and "Headings" were intended to be a submenu,
but due to the broken menus these titles are not used.

    (define-key map [hide] (cons "Hide" (make-sparse-keymap "Hide")))
    (define-key map [hide outline-hide-other]

    (define-key map [show] (cons "Show" (make-sparse-keymap "Show")))
    (define-key map [show outline-show-subtree]

    (define-key map [headings] (cons "Headings" (make-sparse-keymap 
"Headings")))
    (define-key map [headings demote-subtree]

Probably this should be transformed to 'easy-menu-define'.