Create Base FBX for racedata (Optional)

[Jaimi]

Add "Base FBX" to racedata.
If exists, will be used as a base for the race (no base race recipe).
Bones on base FBX will be indexed, and will never be removed. (so easy to setup 3rd party).
Generation will add bones from slots if needed.
Any bones that are not base bones, and that were not added during generation, will be removed.