This is squashed commit (sorry -- we'll get better about maintaining a
clean history in collaborative branches in the future!) of a massive
amount of work from Jim and myself.

* EmailIdentifiers for normal (i.e. not outbox, etc.) emails are the
  same regardless of which folder they came from
* New EmailStore interface to manipulate messages that reside in any
  folder, without having to care what folders are open
* Relevant places that manipulate emails (e.g. the toolbar) have been
  updated to use the new EmailStore interface
* Conversation and ImplConversation have been smooshed together
* Many, many more items and bugfixes related to the above points