Make generated Widget subinterfaces into abstract classes

[JakeWharton]

We want to generate behavior into them. This will also eliminate the annoying override var modifier: Modifier = Modifier boilerplate that every implementation has to do.

[JakeWharton]

This will allow generating a nice toString() automatically which will be useful for leak detection

[JakeWharton]

This doesn't actually solve our problems. Swift lacks abstract classes.

We may have to generate two types instead. A Widget and a Binding. You implement the Binding which is a pure delegate (maybe as a stateless thing), and then Widget wraps an instance + state (like modifier, tree walking) + stateless binding.