(outline-5, outline-6, outline-7, outline-8): New faces.
(outline-font-lock-faces, outline-font-lock-levels): New vars.
(outline-font-lock-face): New fun.
(outline-font-lock-keywords): Use it.
(outline-font-lock-level): Remove.
(outline-mode, outline-next-preface, outline-next-heading)
(outline-previous-heading, outline-next-visible-heading):
Use shy group.
(outline-level) <var>: Update calling convention.
(outline-level) <fun>: Take advantage of it.
(outline-demote): Don't assume the match-data is still uptodate.
(outline-up-heading): Simplify and make sure the match data is
properly set at the end.
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