Add support for date span (2 dates) pasting

[jesus2099]

It's the only remaining unimplemented feature from #20 – make EASY_DATE® better experience

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1. store genuine field values
2. if 1 or 2 dates match in any EASY_DATE field (where 2 dates means setting both begin and end — maybe this requires that we have only one EASY_DATE field to avoid strange behaviours ?)
   1. make it green
   2. assign detected values in genuine fields
3. if no match
   1. keep it normal uncoloured
   2. (re‐)set original values in genuine fields