This is quite a breaking change, but I think it is beneficial due to the large amount of usage of this class.
I originally intended just to remove the allocations of the two delegates handling the `Changed` flow internally, but as nothing was really using the bindables for anything more than a general "point has changed" case, this felt like a better direction.
We've seen multiple cases where DrawableHitObject are stuck in the lifetime management container
due to not implementing a judgement (meaning they are never "hit" or "missed"). To avoid this going forward
CreateJudgement() must be implemented and return a non-null judgement.
This fixes BananaShower and JuiceStreams in osu!catch.
This also makes HitObject abstract and cleans up convert HitObject implementations.