datetime: Rework time zone support in constructors

datetime: Rework time zone support in constructors

Timezone handling is complicated. Really complicated.

In order to simplify it a little bit, we need to expose the GTimeZone
structure.

First of all, we allow creating time zone information directly from the
offset and the DST state, and then pass it to the g_date_time_new_full()
constructor. We also need to clean up the mess that is UTC-vs.-localtime
for the other constructors.

We also allow creating a GTimeZone from the Olson zoneinfo database
names; a time zone created like this will be "floating": it will just
reference the zoneinfo file - which are mmap()'ed, kept in a cache and
refcounted. Once the GTimeZone has been associated with a GDateTime, it
will be "anchored" to it: the offset will be resolved, as well as the
DST state.

https://bugzilla.gnome.org/show_bug.cgi?id=50076