Times respect locales

Times are now formatted according to systems locale.

Beta release: different background color for working hours

• The background of the working hours is drawn in light gray.
• When tapping on the time scale with two fingers for at least 1 second, the working hours are set.
• Additionally working hours may be set via a "conventional" setting on the settings page.

Feature release: recurrence intervals

The event repetition dialogue now provides a field to enter the "recurrence interval" telling the event to repeat only every x-th day/week/month or whatever.