Enable calendar popup for date selection

[VictorLamoine]

Fixes #6

Now when the right arrow is clicked a calendar opens which allows to select any available date:

https://doc.qt.io/qt-5/qdatetimeedit.html#calendarPopup-prop

[VictorLamoine]

@facontidavide friendly ping :golf:

[facontidavide]

I haven't tested it. Does it allow you to select the seconds/milliseconds too?

[VictorLamoine]

Without this merge request the two arrows allows to modify the milliseconds count up/down:

With this modification the single arrow opens the calendar. It is still possible to modify the seconds/milliseconds value by typing in the text area but the up/down arrows are gone.