gtimezone: Add g_time_zone_get_identifier() accessor

This is a non-trivial accessor which gets the identifier string used to
create the GTimeZone — unless the string passed to g_time_zone_new() was
invalid, in which case the identifier will be `UTC`.

Implementing this required reworking how timezone information was loaded
so that the tz->name is always set at the same time as tz->t_info, so
they are in sync. Previously, the tz->name was unconditionally set to
whatever was passed to g_time_zone_new(), and then not updated if the
tz->t_info was eventually set to the default UTC information.

This includes tests for the new g_time_zone_get_identifier() API, and
for the g_date_time_get_timezone() API added in the previous commit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=795165