Make mtfw.anim_retarget an albam custom property

[Brachi]

Animation re-targeting was an experimental feature, where the general bone index of each bone that allows to re-target animations was stored in the custom properties of each edit-bone.

In order to use it everywhere else in a consistent manner, such as to fix #129, it's time to move that property to an albam custom property, just like materials, textures and meshes.