gst/mxf/: Implement parsing of the generic data essence descriptor.

gst/mxf/: Implement parsing of the generic data essence descriptor.

Original commit message from CVS:
* gst/mxf/mxfdemux.c: (gst_mxf_demux_reset_metadata),
(gst_mxf_demux_handle_metadata_generic_data_essence_descriptor),
(gst_mxf_demux_handle_header_metadata_resolve_references),
(gst_mxf_demux_handle_metadata):
* gst/mxf/mxfdemux.h:
* gst/mxf/mxfparse.c:
(mxf_metadata_generic_data_essence_descriptor_handle_tag),
(mxf_metadata_generic_data_essence_descriptor_reset):
* gst/mxf/mxfparse.h:
* gst/mxf/mxftypes.h:
Implement parsing of the generic data essence descriptor.