query: Rework querying/threading machinery

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.

lib/mu-query-threads.hh

@@ -0,0 +1,44 @@
+/*
+** Copyright (C) 2021 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 3, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful,
+** but WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+** GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software Foundation,
+** Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+**
+*/
+
+#ifndef MU_QUERY_THREADS__
+#define MU_QUERY_THREADS__
+
+#include "mu-query-results.hh"
+
+namespace Mu {
+/**
+ * Calculate the threads for these query results; that is, determine the
+ * 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
+ *
+ * @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);
+
+} // namespace Mu
+
+#endif /*MU_QUERY_THREADS__*/