(outline-up-heading): Add `invisible-ok' arg.

(outline-up-heading): Add `invisible-ok' arg.
(outline-up-heading-all): Remove.
(hide-sublevels): Move to end-of-heading before calling flag-region.
(outline-copy-overlay, outline-discard-overlays): Remove.
(outline-flag-region): Use `remove-overlays'.
Don't move to end-of-heading.
(outline-next-visible-heading, outline-back-to-heading)
(outline-on-heading-p): Use outline-invisible-p.
(outline-font-lock-level): Use outline-up-heading's new arg.
(outline-minor-mode): Simplify.
(outline-map-tree, outline-reveal-toggle-invisible): New funs.
(outline): Put a `reveal-toggle-invisible' property.
(outline-level-heading): New var.
(outline-insert-heading, outline-promote, outline-demote)
(outline-toggle-children): New commands.