[PATCH 5/5] Simplify message and headers visibility code in notmuch-show view.

Before the change, headers and message visibility functions took extra care to correctly set `buffer-invisibility-spec'. This was needed because headers overlay `invisible' property had only headers' invisibility spec. So visibility of headers was determined only by the headers invisibility

[PATCH 5/5] Simplify message and headers visibility code in notmuch-show view.

Before the change, headers and message visibility functions took extra care to correctly set `buffer-invisibility-spec'. This was needed because headers overlay `invisible' property had only headers' invisibility spec. So visibility of headers was determined only by the headers invisibility