applets/digitalclock: Localize custom date format, too
Qt.formatDate with locale only accepts Locale.FormatType as format type, no Qt.DateFormat (ISODate) and no format string. Locale.toString on the other hand only formats a date with time...
Also refactor it to store a formatter function which is then used to format the date.
- Long format is localized long date (still working)
- Short format is localized short date (still working)
- ISO date is ISO date (still working)
- Custom date format is now formatted according to locale
- Preview in settings works, too
- Changing date format in settings is reflected live on apply
- Editing custom date format in settings is reflected live on apply