We should also persist mod options, but those will go in a separate file
because they aren't a part of Settings.
Plugins need some work before that can be saved here. They're not
configurable in the NGW currently, so that needs to be fixed first. It
also appears that it may not be safe to inject the settings object with
plugin options until the game is created, but that needs more
investigation (see the comment in Settings.save_player_settings).
Another obvious candidate would be the desired player mission duration,
but we need to implement custom serialization for that first.
This really shouldn't need to happen but I don't feel like rewriting the
culling code right now. There's no reason for these to be persisted to
the Game at all, we should be generating these once they're needed.
