improvement: have a single Robot scene for visuals and practices

[cogwoz]

This PR replaces each visual that uses the robot sprites with a dedicated robot scene which is easier to maintain and add to.

fixes #249
fixes #260

Previously, there were several different versions of the robot because many of the lessons were created in parallel.

There are currently 6 animations, some placeholder:

• idle (default)
• jump
• heal
• damage
• level
• saying_hi

The robot uses an AnimationTree to control its animations.

To play an animation, use the following. The robot will automatically go back to its "idle" animation.

onready var _animation_tree := find_node("AnimationTree")

func _ready():
	_animation_tree.travel("jump")

The AnimationTree has a helper script to allow the above which makes moving to each animation easier.

## RobotAnimationTree.gd
extends AnimationTree

signal animation_finished

onready var _state_machine = self["parameters/playback"]
onready var _animation_player = get_node(anim_player) as AnimationPlayer

func _ready() -> void:
	_animation_player.connect("animation_finished", self, "_on_animation_finished")

func travel(animation_name: String) -> void:
	_state_machine.travel(animation_name)

func _on_animation_finished() -> void:
	emit_signal("animation_finished")

In Godot, currently AnimationPlayers stop emitting signals when controlled by an AnimationTree, so each animation emits the animation_finished signal via the method track which feeds up to the AnimationTree.

This allows us to wait on animations before continuing in practices.

## IncreaseMaxHealth.gd
# ...
func _run():
	for i in range(2):
		level_up()
		_animation_tree.travel("level")
		yield(_animation_tree, "animation_finished")
	Events.emit_signal("practice_run_completed")

I tested all of the visuals and practices, so everything should just seemlessly work.

[NathanLovato]

I quickly went through the scenes and everything seems to be working well. I'll have a tester go through all lessons again soon too. Thanks for your work!