Improve styling API.

Be more consistent with the other controls' styles.

Change-Id: I2c385774ae780c7341a4f72cf38ecdd6030cc274
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Convert headerDelegate into weekdayDelegate.

This allows us to hide CalendarHeaderModel, as we instead provide a
delegate for each weekday.

Change-Id: I246826bef5993fa5b70ad906acdf5c7854a681ba
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>