Allow easier configuration of skins for different proportions

Currently, the workflow described in [this blog post](http://esotericsoftware.com/blog/Skin-constraints-for-different-proportions) allows one skeleton to have different proportions, but since one transform constraint must be added per bone you want to change the transform, trying to do this in a real project would require creating a significant number of transform constraints, and their positioning is not easily done with the current features. We need to be able to achieve different proportions more easily.

Related posts on the forum:
http://esotericsoftware.com/forum/duplicate-constraint-14489
http://esotericsoftware.com/forum/Draw-Order-for-Skins-17288