Calculate the thread subject, that is, the subject of the (sub)thread _or_
empty if it's the same as the previous subject.
This is for the UI feature of _not_ showing the subject when it's just
repeating from the previous.
Rewrite the query machinery in c++:
- use an MSet decorator instead of the mu-msg-iter stuff
- use mu-query-decider to mark duplicates/unreadable/related messages
- use mu-query-threader to replace the older container/thread code
Algorithm did not substantially change, but the implementation details
did.
Dirk-Jan C. Binnema