Fix stupid Orbinaut bullshit

See merge request KartKrew/Kart!120

It actually uses trailing item's throw speeds instead of its own, oddly enough; I don't want to mess with the gameplay there, so I just gave it its own exception.

- Fix Orbinaut/Jawz not accepting top speed changes (This means back-spam being slower now for real works, and the speed buff on boosters for real works.)
- Make the top-speed buffs for boosters not work for back-spam

Let Grow/Shrink revert itself when it changes to 0, as well as add a function for removing either. Means we don't have to modify every single > 0/< 0 check, and is overall cleaner.

Same here

I doubt this hurts anything, but might as well I guess