Bugfix/datepicker feedback

[KoolADE85]

This PR fixes issues found & reported in the new DatePicker:

• Month labels could be cut-off in some languages; so the dropdown width is now dynamic
• Enter key now activates the calendar popup when focused on the dropdown
• Smarter determination of how to handle numberOfMonthsShown & which months are shown, based on how the user interacts
  • when opening a calendar for the first time, the current month will be "centered" in the list of months
  • when navigating "forward" in the calendar, the current month will now be displayed last in the list of months
  • when navigating "backward" in the calendar, the current month will be displayed first
  • in all cases, when navigating to a month that is already visible (numberOfMonthsShown > 1), do not re-calculate or re-position which months are shown: just keep the current view.

[github-actions]

Thank you for your contribution to Dash! 🎉

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[T4rk1n]

💃