Behavior tree support

[Sergey Lapin]

Implementing animation tree please give some time to implement Behavior tree which makes AI design much more easier and standard. Also decouples many systems making code clean. BTs are something which is easy to implement but gives a lot of power.

[Guilherme Felipe de C. G. da Silva]

@reduz Does the "Transition Graph" item in the roadmap mean "AnimationTreePlayer"?

[Rémi Verschelde]

Is this implemented as expected in AnimationTree? If not, please file a proposal on https://github.com/godotengine/godot-proposals, which is the new tracker for feature requests.