Improve model selector type

[lbwexler]

Turns out our homemade class definition was a little too close to a general function definition, and was capturing that.

This alternative from typefest works better, and seems more standard, although it precludes HoistModels with private constructors, like our new ViewManagerModel. Think that's probably ok.

Other changes here and in toolbox were unrelated, just pickups from the stricter typing.

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