Merge SRB2 master & next (round 1)

Scarily big, but since it's mostly stuff already reviewed for vanilla, I'd say decent requirement for +1 is just to see if it compiles, and maybe double check that there's no issues in-game (notably, I played removing the execversion stuff that we don't need now but may need in the future mostly by ear :V)

"Round 1", because we'll always have to merge SRB2 master/next back in all of the time :p