cubicweb #1615820 rangefacet visual feedback [open]

the vocabulary facets provide a visual feedback : when a term is not present in the resultset, it is striked out.

rangefacet could do the same and grey out the range between the minimum of the original range and the minimum of the range of the current (after facet restrictions are applied) rset. The same should be true for maximum.

done in<not specified>
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