ActivitySheet action button support missing

[KaneCheshire]

Looks like I was too hasty merging in the ActivitySheet stuff, we still need a way to reference the buttons like Alert:

func actionButton(withID id: String) -> Button {
  return Button(id: id, parent: self)
}

It's trivial to add this support in an extension in a test target for now but would be nice to sort this.

It also might be worth having a global let activitySheet = ActivitySheet() instance like Keyboard since there's only one shown on-screen at once.