Fix issues with the new "reverse grand prix" feature and user defined GPs

There is a new feature that allows the user to run a grand prix in reverse mode, no reverse mode, or random reverse. It's a nice feature, but unfortunately clashes with the possibility of the user being able to define its own grand prix with some tracks in reverse and some in normal mode. But the solution is simple: adding a fourth option (default), which leaves the reversibility of the tracks in the grand prix untouched.

I have also changed the names in the spinner. "All" and "None" instead of "Yes" and "No", which I think are more clear and less prone to confusion.