Implement Entity: Monster
This MR implements basic functionality for monsters. Also we have improved the view objects cache with this MR to be handle to handle more entities in the current view.
We have introduced a virtual class Behaviour
which can define the behaviour of monsters.
Currently we have implemented SimpleBehaviour
which make the monster roam around it's spawn position
in random timespans and to random targets.
The monsters currently do not support attacking nor getting attacked.