higher-level app shim

[david-vanderson]

@MichaelBelousov has been building an app on top of dvui that functions with multiple backends, including web. One of the pain points is that the web stuff was not written in a way that makes building on top of it work well. In particular merging dvui changes is annoying.

This issue is about a possible way to make this better by adding a higher-level "app" api that inverts control of the mainloop. The app code would expose a few functions (init/deinit/frame) and pass them to some dvui "app runner" interface.

The advantage is more separation between app code and dvui code. The disadvantage is less control over the mainloop.

To be clear this would only be an addition - an extra way to structure apps where this tradeoff makes sense.