IMAP_UNREAD_MAIL should be emitted when new mail is processed too

I noticed that IMAP_UNREAD_MAIL event is only signaled when a soledad sync succeeds, but we want it to be emitted also when new mail has been processed and saved locally.

(from redmine: created on 2013-08-31, closed on 2013-09-02, relates #3662 (closed))