[Engine, Frontend*] Sort participant list in frontends, not completer

To allow for more options when ordering nicknames for completion, move the
sorting of the participant list from NickCompleter to the frontends. This
change is being made in anticipation of support for last-spoken-first
ordering.
[Engine|Frontend-(GNOME|STFL)] Implement last-speaker-first completion ordering (fixes #558).

Allow ordering of completion suggestions by who spoke last. Useful for those
pesky idlers. ;-)