Preview padding is bound to preview spacing

[flekschas]

It seems like the preview padding is bound to the preview spacing:

https://github.com/flekschas/piling.js/blob/master/src/library.js#L942

So I assume it's not possible to have zero spacing but still show a border on hover. Therefore, changing the z-order as requested in #146 might not actually change anything.

I suspect we need to decouple previewSpacing from the padding by introducing a new property called previewPadding. Such that on hover, a border is drawn that might superimpose other previous during the hovering.