Add first_weekday support for Datebox

The date pickup widget always show Sunday as the first day of a week. For users who treat Monday as first day of a week, it's so disordered (7 1 2 3 4 5 6).

In linux OS, I can change first_weekday and/or first_workday in /usr/share/i18n/locales/LANGUAGE_FILE, but I can't do this in ZK. Wish this can be supported in future ZK versions.

just be suprised that no one post a feature request for this before.