Skeleton for 2D characters and animated objects. Skeleton2D parents a hierarchy of [Bone2D] objects. It is a requirement of [Bone2D]. Skeleton2D holds a reference to the rest pose of its children and acts as a single point of access to its bones. $DOCS_URL/tutorials/animation/2d_skeletons.html Returns a [Bone2D] from the node hierarchy parented by Skeleton2D. The object to return is identified by the parameter [code]idx[/code]. Bones are indexed by descending the node hierarchy from top to bottom, adding the children of each branch before moving to the next sibling. Returns the number of [Bone2D] nodes in the node hierarchy parented by Skeleton2D. Returns the [RID] of a Skeleton2D instance.