Add target bone selection for rootmotion retargeting

[smix8]

Currently rootmotion only works when both skeletons have the root bone as the first (root) bone in the skeleton.

If one skeleton has root motion on the first one, e.g. mixamo hip bone, and one skeleton has a static root bone with a moving, second hip bone retargeting the source hip to the target hip wouldn't work.

A custom mapping can not solve this issue because all positions are relative to the root and one is moving while the other is not. Also Godot forces bones to start at the end position of their parent bone so the static root bone length and orientation needs to be considered as well.

This would partially fix issues seen in #19

[smix8]

Added to the feature tracker.