Generate abstract class which does mutable state binding for Compose UI

[JakeWharton]

For convenience

abstract class AbstractComposeUiButton : Button<@Composable (Modifier) -> Unit> {
  private var _label: String? by mutableStateOf(null)
  protected val label: String get() = _label as String

  @Override public fun label(label: String) {
    this.label = label
  }
}

Related to #2619

[JakeWharton]

So the design in #2749 ends up using composable lambda functions, but this incurs boxing. So we need to change it to maybe

abstract class AbstractComposeUiButton : Button<@Composable (Modifier) -> Unit> {
  private var label: String? by mutableStateOf(null)

  override val value: @Composable (Modifier) -> Unit = { modifier ->
    Button(
      label as String,
      modifier,
    )
  }

  @Composable
  abstract fun Button(label: String, modifier: Modifier)

  @Override public fun label(label: String) {
    this.label = label
  }
}

[JakeWharton]

I updated to a list of abstract functions on an abstract base type for now.

[JakeWharton]

Need to migrate existing bindings to use this