clock: update every second

If we don't update every second, we may show the wrong time for up to
a minute on 1. resume; or 2. when changing the time; or 3. when
changing the timezone. This is both annoying and and leads to people
thinking that the tool for changing the time / timezone is broken.

https://bugzilla.gnome.org/show_bug.cgi?id=635840



Signed-off-by: David Zeuthen <davidz@redhat.com>