Property change states

Use case: ``. The `currentSlide` is currently a "state" value and this is preferable. We could consider making it a controlled property coupled with `onSlideChange` callback, but that'd expand the API surface for something very simple. However, what should happen if the `currentSlide` prop (or "current-slide" attribute in DOM) has been updated? The simplified [useStateFromProp](https://github.com/ampproject/amp-react-prototype/blob/104861c3777d5341377092620f123b9013c0050d/src/amp-react-utils.js#L32) hook supports updating the internal state when a prop changes, but it's not without issues. For instance, assuming the default prop/attribute value of `currentSlide = 0`, if the `currentSlide` state has been updated and the script simply wants to reset it back to the first slide, setting the `currentSlide` back to `0` will not work since technically the property's value will not change.