Added 24h clock mode to the analog clock.

It is a bit frivolous but the changes are surprisingly small (all 24h underpinnings are already available) and there is a passionate community of 24 hour watch lovers out there.

Please decide whether to ship it or not, no hard feelings if you choose not to.

(If you have received this MR twice, that's because I've messed up the previous one - used a wrong branch as a reference).