Allow subclassing AnimatedSwitch and Button

[jandamm]

In order to create a AnimatedSwitch which applies the
traitCollection either a subclass of AnimatedControl and
reimplementing all the switch logic or creating a wrapper view and
exposing the AnimatedControl has to be done.
Same is true for AnimatedButton.

final class ThemedAnimatedSwitch: UIView {
    let toggle: AnimatedSwitch
    let redValueProvider = ColorValueProvider(Color(r: 1, g: 0.2, b: 0.3, a: 1))

    init(frame: CGRect) {
        toggle = .init(animation: animation)
        super.init(frame: frame)

        toggle.setValueProvider(valueProvider, keypath: AnimationKeypath(keypath: "**.Fill 1.Color"))
    }

    required init?(coder: NSCoder) { ... }
}

Then use animatedSwitch.toggle.

---

final class TestSwitch: AnimatedSwitch {
    let redValueProvider = ColorValueProvider(Color(r: 1, g: 0.2, b: 0.3, a: 1))

    convenience override init() {
        self.init(animation: animation)

        setValueProvider(valueProvider, keypath: AnimationKeypath(keypath: "**.Fill 1.Color"))
    }
}

Then use animatedSwitch directly.

Since the public interface of AnimatedSwitch is public instead of
open functions like func setIsOn(_ isOn: Bool, animated: Bool, shouldFireHaptics: Bool = true) cannot be overridden.

Supersedes #1359

[jandamm]

@calda please have a look :)

[calda]

Thanks!