Save skin names in save files instead of skin numbers

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.)