Climbing improvements

#478 I tried to fix the climbing as good as I could, however my math skills are not good enough for a really elegant solution. While I guess this could be solved a lot smarter for someone with the skills, it's still a major improvement over the current climbing behavior.

Basically the following changes were made:

- Climbing initiates only when touching the ladder. Previously climbing was also initiated when you were a good step away from the ladder, which felt weird.
- While Climbing, Movement is always restricted to 2 axes, depending on how you are facing the ladder (see the 3 modes below). Previously, when you hit the back-key while facing the ladder, you moved down the ladder AND backwards, which eventually detached you from the ladder.
- Climbing Mode 1. facing the ladder or having a pitch > 60 Degrees : forward/back will move up/down,  strafe left/right will strafe left/right (similar to the way it worked before)
- Climbing Mode 2. Looking sidewards relative to the ladder: forward/back will move forward/backward, strafe left/right will climb up/down
- Climbing Mode 3. looking away from the ladder: forward/back will move forward/back, strafe left/right will move left/right (this mode doesn't allow you to climb, but  allows to detach yourself from the ladder by just moving in the opposite direction from the ladder, which is not possible in the other modes)