Add support for calculating civil twilight times

This is needed for example to implement the OSM opening hours specification
(https://wiki.openstreetmap.org/wiki/Key:opening_hours), as attempted in
https://invent.kde.org/vkrause/kopeninghours.

The existing sunrise code has been refactored to support this, and it
should now be very easy to add support for other sun position times such
as astronomical or nautical twilight as well, should that ever be needed.