event-editor-schedule-section: redesign

This MR implements parts of the new designs for the schedule section of the event editor.

Here is the current look in Timeslot mode:

It introduces two new widgets: DateChooserRow and TimeChooserRow.

Details:

• DateChooserRow:
  • Editable as an EntryRow
  • The parser is the same as in main
  • The popover is the same as in main
  • The formatter to display a date is the same as in main
  • If a valid date is typed, unfocusing the DateChooserRow will set the date and format it correctly (like in main)
  • If an invalid date is typed, unfocusing the DateChooserRow will reset the date to its precedent value (like in main)
  • Add accessibility label to button
  • Add tooltip to button
• TimeChooserRow:
  • Editable as an EntryRow
  • In 12H format, the parser accepts:
    • [required] a number between 0 and 12 (padded or not) for the hours,
    • [optional] followed by a separator* and a number between 0 and 59 (padded or not) for the minutes,
    • [optional] followed by a space
    • [required] followed by am or pm (case insensitive) (uses translation)
  • In 24H format, the parser accepts:
    • [required] a number between 0 and 23 (padded or not) for the hours,
    • [optional] followed by a separator* and a number between 0 and 59 (padded or not) for the minutes,
  • Add accessibility label to button
  • Add tooltip to button

*Allowed separators are "-", ":", ".", "/", "_", ",", "'", any uppercase/lowercase letter.