Refactor move not-OpenSim-related classes out of `OpenSimBindings/Tabs`

[adamkewley]

Estimate: 1-2d (depends how easy it is to source an obj parser and refactor MainUIScreen)

The following screens/tabs don't strictly require OpenSim:

• MainUIScreen
• MeshHittest
• GeometryShader

The reason they are in there is because they use SimTK's obj loader. If a not-OpenSim-or-SimTK obj loader could be integrated then we could move these into the more general part of the codebase.

[adamkewley]

Dropping this from 0.4.0 because it's a cleanup that should be done in 0.4.1 (the "tock" to the feature-heavy "tick" of a major release).

[adamkewley]

Moving this to 0.5.0 or beyond because 0.4.1 will include separating the engine+OpenSim parts of the application (the refactor proposes effectively moving those UI things from OpenSim to the engine, which should be done after the split).

[adamkewley]

Better for 0.5.1: not MVP for frame definition