NormalViewState appstate and hook up for SceneViewer

[neph1]

This is using a temporary icon. I'll fix that once #385) is merged, rather than building this on top of the other branch.

Normal length could be debated. Currently it's 0.2 units.

Fixes #200

[neph1]

Maybe it should have support for moving stuff around. Currently you'd have to toggle it off and on to see the changes.
Maybe also a refactoring of SceneApplication should be considered if we add more functionality, moving more to AppStates.

[neph1]

Now it updates the debug transforms every update. Unnecessarily expensive? I considered doing some checks, like debug.transform != geometry.transform, but the checks themselves might be just as expensive.
I'd like to check for if a geometry is added or removed and mirror that in the debug view, but while I thought checking for removed ones would be simple, it didn't work as I had expected.