Sort by thread date

Recent discussion on the Balsa mailing list (https://mail.gnome.org/archives/balsa-list/2018-August/msg00000.html) focused on an annoying feature of Balsa's sorting of the index of messages in a mailbox. Clicking on the 'Date' column header sorts the index by date, which in the case of a threaded view is the date of the head message. A recent reply in an old thread can be hard to find, because the thread is sorted to a distant position. In the new 'thread-date' branch, the sorting algorithm is changed so that threads are sorted by the date of the most recent message in the thread. The date displayed for a message is not changed, of course, only the position of the thread in the index. As implemented, this is not an option; I view it as a fix for a long-standing flaw. I suppose it could be made optional, with a check-button somewhere in the preferences, if there really is a need for the previous behavior.