emacs: Use result text properties for search result iteration

emacs: Use result text properties for search result iteration

This simplifies the traversal of regions of results and eliminates the
need for save-excursions (which tend to get in the way of maintaining
point when we make changes to the buffer).  It also fixes some strange
corner cases in the old line-based code where results that bordered
the region but were not included in it could be affected by region
commands.  Coincidentally, this also essentially enables multi-line
search result formats; the only remaining non-multi-line-capable
functions are notmuch-search-{next,previous}-thread, which are only
used for interactive navigation.