gst: (de)serialization of GLibDateTime so GDateTime properties can be set via gst-launch-1.0
This merge request allows gst-launch
to accept iso date stamps for GDateTime properties.
For example using timeoverlay with a reference timestamp in timestamp/x-unix
rather than timestamp/x-ntp
timeoverlay time-mode=reference-timestamp show-times-as-dates=true datetime-epoch='1970-01-01T00:00:00'