Never autosave over a user's files

We should only ever be autosaving to the single autosave file.
Never claim the engine has died when all is well

Currently we display an erroneous "Oops! Something has gone wrong" if
you start a new game within two seconds of having completed the previous
game.

Incindentally fixes https://bugzilla.gnome.org/show_bug.cgi?id=721671
There is no computer player if engine is not found

Confusingly, human_player == null means there are two humans.
Fix critical warnings when starting a new game

Never try to remove the same source twice
Fix figurine algebraic notation even better

You'd think I would have noticed the colors were wrong when I fixed the
pieces. You'd think wrong!