Experimental "ondemand"-loading of thumbnails.
 The option `thumbnails: 'ondemand'` can be set in order to load thumbnails  when needed:
  * Thumbnails that are in the visible range of the thumbnails-view are loaded
  * Thumbnails, that are adjacent to the visible range (one carousel-width before or after) are loaded.
  * As the carousel moves, more thumbnails are loaded.
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