This avoids us having to poll once a second.  Because GSource isn't
introspectable, we need to make a C utility class.

Note that GTimezoneMonitor was never used in gnome-shell; here we also
add an inotify watch on /etc/localtime.

Thus the clock should update more or less instantly after:

1) A minute expires
2) The schema changes
3) The system realtime clock is modified discontiguously
  - Including resume from suspend
4) The system time zone changes

https://bugzilla.gnome.org/show_bug.cgi?id=657074