Support Sets as selected dates

[jclaveau]

This PR allows to use Javascript's Sets as selected-dates to handle uniqueness automatically if wanted.

• https://developer.mozilla.org/fr/docs/Web/JavaScript/Reference/Global_Objects/Set

I placed it into useCommon as it seems to be the natural way it do it following your example

Please tell me if this features seems interesting to you and if you know a better way to implement it. Thank you very much for your work!