Aggregate some more thread columns properly.

- show thread as starred if at least one message in it is starred
- use arrival date of last message as thread arrival date
- use sum of message attachment counts in thread as thread attachment
  count
- show thread as answered/forwarded if all messages in thread were
  answered/forwarded
Merge pull request #240 from maniac103/thread-star

Show thread as starred if at least one message in it is starred.