Level does not go lower than the deathTreshold

Also moded the deathTreshold to P since it's needed by more than one
class now.
Improved checkOnGround

playerControl.checkOnGround() now does two raytests, one close to the
front edge of the model and one close to the back end. Prevously id did
only one, in the middle, which made the player get stuck platform edges
sometimes.