Add `Point Forces` and `Point Moments` force visualization

[adamkewley]

The existing body force visualization already "resolves" point forces to be on bodies. This is what OpenSim::Force and simbody ultimately want.

However, many use-cases involve the user adding point-forces into a model w.r.t. some frame (e.g. an ExternalForce typically works this way). The user's going to find it very confusing when they see their force vector has moved+changed to the body - even if that's ultimately what the engine "sees".

So, as an additional visualization, the UI should also supply a way of viewing the point forces. This isn't exposed by OpenSim's Force class, so OSC will have to specialize (downcast) it for OpenSim::GeometryPath and OpenSim::ExternalForce