lib: improve threaded sorting

Improve threaded sorting:
- rework the recursive algo (which should a crash noted earlier)
- sort by thread key (as in 1.5.7)

Fixes: #1923
Fixes: #1922
Fixes: #1911
Fixes: #1916

lib/mu-query-threads.hh

@@ -28,16 +28,13 @@ namespace Mu {
  * thread-paths for each message, so we can let Xapian order them in the correct
  * order.
  *
- * Note - threads are can be order by an arbitrary field for the top level, but
- * the messages below the top level are always sorted in chronologically
- * ascending orde
+ * Note - threads are sorted chronologically, and the messages below the top
+ * level are always sorted in ascending orde
  *
  * @param qres query results
- * @param sort_field the field to sort the top-level by
  * @param descending whether to sort the top-level in descending order
  */
-void calculate_threads (QueryResults& qres, MuMsgFieldId sort_field,
-                        bool descending);
+void calculate_threads (QueryResults& qres, bool descending);
 
 } // namespace Mu