* Added helper functions to Skeleton3D for converting transforms from bone space to global space, and vice versa.
* Updated the Skeleton3D class reference.
* Changed the icon used for bones in the Skeleton3D inspector to use BoneAttachement3D's icon.
* Changed the Skeleton3D inspector to use EditorPropertyTransform and EditorPropertyVector3 when possible.
* Placed the Transform/Matrix for each bone in a sub-section, so it is visually similar to the Node3D inspector.