Add ability to select which frame to edit quantities w.r.t. in mesh importer

[adamkewley]

Related #33

A user on LinkedIn is building a mechanical model in solidworks and wants to import their meshes into OpenSim. They used the mesh importer to do this (although, it might be that the model editor is a better fit) and found it frustrating that the quantities in the mesh importer are all expressed w.r.t. ground, whereas solidworks (appears to?) express quantities w.r.t. specific frames in the model (local space?).

Therefore, it would be useful to have a feature such as:

• Add a dropdown to the editing context menu in the mesh importer called "Quantities w.r.t." or similar (better name needed)
• The dropdown should default so "global coordinate system", "ground", or similar
• The dropdown will let the user select a body/joint/mesh in the mesh importer that they've already added
• If the user selects one of those, the translation and orientation editors in the menu should then be expressed w.r.t. the thing that the user selected