Add ability to add stations/points of interest via the osim editor

[adamkewley]

Estimate: 2-5d

This might require a little coercion (picking relevant body frames etc), but it was a commonly-requested feature in the workshop.

Effectively, users would like to be able to define 3D locations in the osim editor such that they can use those 3D locations in other OpenSim components (e.g. muscle path points).

The maths etc. for this is easy enough but I'll need to explore how the overall UX should play out. Coordinates in the osim editor are always relative to something, so it might be that we first ask the user to place the point somewhere (by clicking) but then need to temporarily transition to a modal state in which the user selects what frame that location is defined in (required if they are eventually going to be using it to place muscle points etc.)

[adamkewley]

The latest model editor 3D viewport that was shipped in 0.4.0 nearly has the necessary functionality to deliver this, but it needs enhancement to support modal states.

[adamkewley]

Modal state support was added during #490 , but it might be the case that the user being able to reassign a socket without the object moving in the 3D scene (#326) helps alleviate the need for a modal flow.