[notmuch] Rework of attachment saving

[camalot at picnicpark.org]

I think I've reworked the attachment savings to behave as we've been
discussing.  I don't know anything about the button handling so I
haven't attempted to implement the direct manipulation approach of of
saving using the buttons.  That would certainly be nice to have
however and I belive this change should make it easy for someone who
knows how buttons work to implement it.

     --- Keith