Per player difficulties

This time it's a bigger pull request, that introduces "per player difficulties".
This allows players to change the properties of their karts relative to the other karts. For example the karts can be boosted so they are faster and items have less negative (but also a greater positive) effect.
This can be done in the player configuration and is activated for multiplayer games but can also be activated for all singleplayer games, except story mode.