emacs: show: add overlays for each part

emacs: show: add overlays for each part

This makes notmuch-show-insert-bodypart add an overlay for any
non-trivial part with a button header (currently the first text/plain
part does not have a button). At this point the overlay is available
to the button but there is no action using it yet.

In addition the argument HIDE is passed down to
notmuch-show-insert-part-overlays to request that the part be hidden
by default but this is not acted on yet.