RandomTextureSpriteBehavior: Allow idle + walk animation

Assets:

- Rename existing part assets to "idle".
- Add walk animation for the legs part.

RandomTextureSpriteBehavior:

- Export an Array of SpriteFrames resources. Instead of using the custom addon
  to change the textures.

Townie scene:

- Add a CharacterSpriteBehavior to each part. And configure the legs part with
  the new animations.

- Add one .tres file per body part variant. And configure each Array of
  SpriteFrames resources with them.

Townie demo:

- Add a FollowWalkBehavior to one townie, and remove the TalkBehavior.

Author: manuq

scenes/game_elements/characters/components/character_randomizer.gd

@@ -131,6 +131,7 @@ func _randomize_all_sprites_progress() -> void:
 	var random_frame := randi_range(0, frames_length)
 	var random_progress := randf()
 	for sprite in animated_sprites:
+		sprite.play("idle")
 		sprite.set_frame_and_progress(random_frame, random_progress)