display a collection in a Bootstrap carousel

This PR allows to display collection items in a Bootstrap carousel.
Unlike Galleria (used in the existing Carousel tile) , the Bootstrap carousel is not restricted to images only, it can show any HTML content as slides.
That's the only reason why I made this change.
Nevertheless it might be quite confusing for users if 2 different tiles offer a carousel feature in 2 diferent ways...
So I am not sure it must be merged, but it is a way to launch a discussion about it.