[SAFETY COMMIT] Freeze changes to vertical velocity.
Since other fixes migth be in focus, some changes are commited
merged/commented out:
* Prepare AxisVelocity to carry positive and negative values.
* Prepare MovingData to use vertical AxisVelocity. Rename hVel.
* Prepare CreativeFly (...), do not count velocity for height limit.

(Most preparations were unfinished, just hinting at the direction.)