Try really hard to not stack games in history

I don't have time to remove the history class before 3.9.90.  In the
mean time, let's just make very sure that we never have more than one
game in history by deleting everything before adding one.

Obviously a massive FIXME, but the only thing the user will notice is
that autosave/autoload works as expected.

https://bugzilla.gnome.org/show_bug.cgi?id=705878