Support specifying calendar's default date

Summary:
The --default option can now be used to specify a default date for the
calendar widget. The given date has to follow the format specified with
--dateformat, or, if missing, the default format ('ddd MMM d yyyy').

Users may want to specify an initial date other than 'today'.

Reviewers: #kdialog, dfaure

Reviewed By: #kdialog, dfaure

Differential Revision: https://phabricator.kde.org/D29778