Slot delegation

Consider a carousel button customization. There are two approaches to place it in carousel:
1. Clone it as a vDOM component. This has a problem of losing styles, external event handlers, and an expected DOM hierarchy.
2. Slot it. The problem is that `disabled` attribute (Web spec issue) and event handlers (React issue) do not propagate from the `slot` to the distributed button.

As a result we created the [implementation](https://github.com/ampproject/amp-react-prototype/blob/c4c7373542f30feda4a1b539db1781f88bdc5063/src/react-compat-base-element.js#L605) where we propagate state and events from the slot to button and back manually.