Orientation tags need to be transformed from the string tag returned
by gstreamer to the orientation in degrees. Date tags need to be
inserted with gst_date_time_new_local_time. Finally setting a tag
value shouldn't clear all other tags.
Change-Id: I28922148251084c12cf6c93d9b097fa5df41da9d
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>