Add GST_TYPE_DATE, a boxed type that wraps GDate, and the usual bunch of utility functions along with a hack that che...

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gststructure.c: (gst_structure_set_valist),
(gst_structure_get_date):
* gst/gststructure.h:
* gst/gstvalue.c: (gst_value_set_date), (gst_value_get_date),
(gst_date_copy), (gst_value_compare_date),
(gst_value_serialize_date), (gst_value_deserialize_date),
(gst_value_transform_date_string),
(gst_value_transform_string_date), (_gst_value_initialize):
* gst/gstvalue.h:
Add GST_TYPE_DATE, a boxed type that wraps GDate, and the usual
bunch of utility functions along with a hack that checks that
developers don't accidentally use G_TYPE_DATE where GST_TYPE_DATE
is required. Part of the grand scheme in #170777.