Global observers

[mrchantey]

An overhaul of beet_flow, beet_spatial and beet_ml to use global observers.

• closes FallbackFlow in Behaviour Tree should be called Selector #30
  • docs have been updated to reflect fallback / selector similarity
• closes OnRun Context and Global Observers #27
  • this is the feature mentioned in that pr
• closes Fix despawning observers #23
  • observers no longer have lifecycles
• closes Performance with lots of entities #22
  • actions with global observers have a 5x add/remove performance compared to local ones, and this pattern allows for shared trees, entity pools etc
• closes Automatic observer and system registration #18
  • action observers are now registered via component hooks, systems must now be registered like normal
• closes TargetAgent Relations #10
  • TargetAgent is now deprecated