deleteBoneJoint undo is incomplete (doesn't track resulting geometrical transformation)

[m-7761]

The following matrix code isn't passed to the undo system:

https://github.com/zturtleman/mm3d/blob/8f121241275d3f52dcd0eaab6ef846c8c0b23ded/src/libmm3d/model.cc#L952-L962

I noticed this when wanting to add a "Parent" item to the Properties panel. In the following code I solved it by moving the body into setBoneJointParent, and having it pass the new relative matrix to a new private method called parentBoneJoint that adds the new and previous relative matrices to the undo object, that in turn uses "parentBoneJoint":

m-7761@21f3ce4

This also makes setBoneJointParent useful for actually reparenting joints as a user API.

[m-7761]

WARNING: I remembered the next day that many loops are written with the assumption that parent nodes are lower in the joint list, so that if reparenting is permitted to break this assumption, something major has to be done about it; a full audit, algorithm, etc. I suppose I will do that shortly.