query.cc: allow to return query results unsorted

Previously, we always sorted the returned results by some string value,
(newest-to-oldest by default), however in some cases (as when applying
tags to a search result) we are not interested in any special order.

This introduces a NOTMUCH_SORT_UNSORTED value that does just that. It is
not used at the moment anywhere in the code.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>