Minimizing freezes when loading new floor

Made the following changes to minimize freezes when loading a new floor:

• Loading only one robot per frame
• Slowing down player while loading
• Blocking player for reaching the end of the stairs, if robots are still loading

In normal conditions players should never reach the block at the end of the stairs, they should only feel a slight deceleration.
But if for some reason robots are still loading the player will be stopped at the end of the stair so they can't see the robots loading.