emacs: insert quotable parts in reply as they are displayed in show view

emacs: insert quotable parts in reply as they are displayed in show view

In reply, insert quotable parts using notmuch-show-insert-bodypart
instead of calling notmuch-mm-display-part-inline directly to render
the quoted parts as they are rendered in show view.

We use a temp buffer to not leak text properties from the show
renderer into the reply. This way we also don't need to worry about
narrowing or point placement. Credits to Mark Walters
<markwalters1009@gmail.com> and Austin Clements <amdragon@MIT.EDU> for
getting this part straight.

The notable change is that replies to text/calendar parts quote the
pretty printed output of icalendar-import-buffer rather than the ugly
raw vcalendar.