Add shortest-path relative positioning in the mesh importer

[adamkewley]

Estimate: 2-4d

Some users prefer positioning things in the local coordinate system. This is particularly handy for users that have their frames set up in some external software that happens to use a different coordinate system.

"Shortest path" refers to the fact that a body may be joined to ground by multiple joints. The "shortest" path should be chosen when deciding which parent frame to use for this local-space transform.