Added preemption mode to Queuebot

Preemption mode forces Queuebot to always ask subclasses for an action. This
allows subclasses to continue monitoring the environment while executing an
action sequence, and react to it as needed.
Small changes and bugfixes

- Changed color of sight range indicator ring
- The max walk distance ("step") a realbot can travel in a single frame is now
  given to virtualbots at initialization. Queuebot assumes a step if no
  arguments are given in queue_walk()
- Realbot sight range raised to 100 for testing purposes. Stalkerbot really op.
- Fixed an OBOB in Navbot causing it to fail to navigate to locations right
  next to walls.