The return value is nullable, but gnome-calendar fails to check it.
Because the nullable return value is inconvenient for us, we can instead
add and use gcal_context_get_icaltimezone() to supplement the existing
gcal_context_get_timezone().
We'll also need a function to convert from GTimeZone to ICalTimezone,
gcal_timezone_to_icaltimezone().
Fixes#419