Writes skin names instead of skin numbers into SP save files.
This means that skins don't need to be loaded in a specific order when playing a mod with save games. This also means that allowing save games for character mods would be feasible after this (wink wink, nudge nudge)
This maintains backwards compatibility by writing a special value in the old skin number's place. If it does not match the special value, then it uses the old code. (This value is a define, and all of the backwards compatibility code is behind if the define exists, so it will be easy to remove for 2.3.)