gst/mxf/: "Generation UID" and "This Generation UID" are different so handle them different. Also both are not part o...

Original commit message from CVS:
* gst/mxf/Makefile.am:
* gst/mxf/mxfdemux.c:
* gst/mxf/mxfmetadata.c: (mxf_metadata_handle_tag),
(mxf_metadata_preface_handle_tag),
(mxf_metadata_identification_handle_tag),
(mxf_metadata_content_storage_handle_tag),
(mxf_metadata_essence_container_data_handle_tag),
(mxf_metadata_generic_package_handle_tag),
(mxf_metadata_track_handle_tag),
(mxf_metadata_sequence_handle_tag),
(mxf_metadata_structural_component_handle_tag),
(mxf_metadata_generic_descriptor_handle_tag),
(mxf_metadata_locator_handle_tag),
(mxf_metadata_locator_class_init):
* gst/mxf/mxfmetadata.h:
"Generation UID" and "This Generation UID" are different so handle
them different. Also both are not part of every metadata type.