Implementation of `Carousel`

[markusjwetzel]

<Carousel>
  <Carousel.Indicators />
  <Carousel.Inner>
    <Carousel.Item>
      <Image ... />
    </Carousel.Item>
    <Carousel.Item>
      <Image ... />
      <Carousel.Caption>
        <Text>Slide caption</Text>
      </Carousel.Caption>
    </Carousel.Item>
  </Carousel.Inner>
  <Carousel.ControlPrev />
  <Carousel.ControlNext />  
</Carousel>

vs.

<Carousel>
  <Carousel.Item>
    <Image ... />
  </Carousel.Item>
  <Carousel.Item>
    <Image ... />
    <Carousel.Caption>
      <Text>Slide caption</Text>
    </Carousel.Caption>
  </Carousel.Item> 
</Carousel>

We somehow need to determine the number of slides to generate the indicator elements. This seems to be easier with the last approach.