WIP: store message headers in document data area

[David Bremner]

Every Xapian document (thing in the database) has a data area that
stores an arbitrary string. That string is not usable for searching
(unlike terms or values), but can be used e.g. for display. Currently
we don't use this part of the Xapian document at all.  While I was
working adding all of the subjects of multi-file messages for regex
search, it seemed like it might be useful to fix the default subject
displayed to the user independent of how search terms/values are
added. I also have the feeling this might be useful for encrypted
headers. Before I suggested that I wanted to get a clearer idea of how
hard that would be to do.

There's really not that much new code here, it's mainly tests, and
some code movement. Unlike message properties, these are not currently
backed up. I guess that could be done, although I'd like to have a
clearer idea of the use cases before I do that.