Made low level changes to the Skeleton3D class and Skeleton3D inspector. Changes listed below:

* 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.

editor/editor_properties.h

@@ -392,6 +392,8 @@ protected:
 
 public:
 	virtual void update_property();
+	virtual void update_using_vector(Vector3 p_vector);
+	virtual Vector3 get_vector();
 	void setup(double p_min, double p_max, double p_step, bool p_no_slider);
 	EditorPropertyVector3(bool p_force_wide = false);
 };
@@ -536,6 +538,7 @@ protected:
 
 public:
 	virtual void update_property();
+	virtual void update_using_transform(Transform p_transform);
 	void setup(double p_min, double p_max, double p_step, bool p_no_slider);
 	EditorPropertyTransform();
 };