[Example App] stress testing

Do more with the example app.

It would be great to have some kind of automation like the IGListKit "stress test". We could also run this as part of the UI Test suite.

## Initial steps

- Introduce a new `NumberModel`. Similar to the existing `ColorModel` but it just wraps an `Int`. The motivation here is to be able to easily generate very large data sets. (And keep things simple and easy.)
- Add a new section with `NumberModel` objects to the existing views (just have like `0` to `9` or something small)

## Later steps

- In a new tab in the example app, generate thousands of `NumberModel` items
- Implement some automation to shuffle them around (like the existing views, but automatic)

## Other ideas (for later)

- Shuffle/Add/Remove items
- Shuffle/Add/Remove sections