Datetime parsing automatically converts the timezone discarding the original one

[konstin]

Currently GLib doesn't have a way to create a DateTime object exactly representing a iso8601 timestamp. Instead, the parsed timestamp is converted to utc. The conversion itself is valid, but the original timezone is lost.

There is an implementation of the required functionality in gstreamer, implemented here, yet their datetime struct diverges from the GLib one. Instead of fixing the glib it is also be possible to implement the missing functionality in liboparl.