GdaTime: ported to GDateTime; default to UTC
GdaTime now internally holds a GDateTime, set to epoch date 1970-1-1. When saving data to data base it defaults to UTC time, because that is the default on SQLite and may other applications. Added new API to GdaTime, so for UI is possible to show time always in local time, UTC or with time zone. Is possible to manipulate time using GTimeZone.
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