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 spec.  The patch sets
headers overlay `invisible' property a list with both the headers
and the message invisibility spec.  This makes headers invisible
if either of them is added to the `buffer-invisibility-spec' and
allows to simplify the code.