I have made some improvements to the display-time-world UI.  I divided
them up into four patches to ease review and merging of the individual
features.  Please let me know what you think.

(Of course I can squash the patches before pushing if that is preferable.)

Patch 4 adds an alias 'world-clock'.  Ideally, I would like to rename
the somewhat obscurely named 'display-world-time' to 'world-clock' and
make the old names into obsolete aliases.  It would be good to hear
any opinions on that too.

I'm also not sure if any of this should go into NEWS, so I didn't do
that work for now.  Let me know if that's desirable.

Best regards,
Stefan Kangas