Add point-based frame definition support to the mesh importer

[adamkewley]

Similar to how the specialized "Frame Definition"

• Allows users to define edges in the mesh importer
• Allows users to define frames w.r.t. those edges
• "Bakes" the definitions into a standard opensim model format on export (e.g. elildes the edges, which aren't supported)
• Has support for saving framedefs.toml, for warping workflows
• Saves to .oscmis, rather than osim, if the user wants the software to actually remember the edges etc (atm, it'll bake an osim file, which isn't quite right)

[adamkewley]

This has been delayed because StationDefinedFrame support was natively integrated into OpenSim itself in:

• Add initial implementation of StationDefinedFrame opensim-org/opensim-core#3694

There are a few outstanding issues with the implementation - namely, that it has issues with chained PhysicalOffsetFrames, and those issues are being separately addressed in the (to-be-merged) PR:

• Fix chained pofs or sdfs in model opensim-org/opensim-core#3711

Once those are merged upstream, I have already verified that OSC will be able to use them essentially immediately by building a branch of OSC against those patches:

• Feature station defined frames rolled out to OpenSim Creator #823

So the downstream integration steps of putting frame-definition support into the model editor should be more-or-less done. Adding it into the mesh importer is a separate concern, but will require separate engineering step (some are done against this issue) in order for that to work.