Limit the number of previews by running some clustering approach

[flekschas]

As we did in HiPiler, there should be an option to limit the number of previews we're showing.

This can either be done by simply not showing more than x previews or by employing some clustering approach and showing an aggregated preview.

Either way, we need to provide some indicator for how many items are on a pile or a general indicator showing that there are more items on a pile than shown by the previews. The option could be called indicatePileSize with the options visually and numerically.