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.