* Allow engine arguments to depend on difficulty
* Improve difficulty levels for many engines... but not much
* Stockfish and then Phalanx are now preferred over GNUChess
Unfortunately, all of these engines are too hard for our needs.
Some of these engines seem to be Windows-only; others are proprietary;
and others don't want to compile anymore. I've left these alone since
there's not much reason to remove them in case you do manage to get one
working. But they won't be getting difficulty levels.
https://bugzilla.gnome.org/show_bug.cgi?id=475535
