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.


(cherry picked from commit f3ef11ea)