use text properties rather than overlays

[David Edmondson]

In preparation for changes to support showing and hiding MIME parts on
the fly (general parts, choosing amongst multipart/alternative and
show updated signing/encryption status without redisplay) I'm trying
to clean up some of the `notmuch-show-mode' code. This patch is part
of that, though it stands upon its' own.

Austin provided some code to allow the invisible overlays to be added
to `notmuch-search-mode' buffers lazily. That same approach could be
used here, but it's not implemented yet.

[PATCH 1/2] emacs: Use text properties rather than overlays in
[PATCH 2/2] test: Update test to match previous patch.