Implement support for OP2a/b/c and OP3a/b/c, i.e. tracks with more than a single component. This currently only works...

Original commit message from CVS:
* gst/mxf/mxfaes-bwf.c: (mxf_bwf_handle_essence_element),
(mxf_aes3_handle_essence_element):
* gst/mxf/mxfalaw.c: (mxf_alaw_handle_essence_element):
* gst/mxf/mxfd10.c: (mxf_d10_picture_handle_essence_element),
(mxf_d10_sound_handle_essence_element):
* gst/mxf/mxfdemux.c: (gst_mxf_demux_pad_init),
(gst_mxf_demux_choose_package),
(gst_mxf_demux_handle_header_metadata_update_streams),
(gst_mxf_demux_pad_next_component),
(gst_mxf_demux_handle_generic_container_essence_element),
(gst_mxf_demux_parse_footer_metadata),
(gst_mxf_demux_handle_klv_packet), (gst_mxf_demux_src_query):
* gst/mxf/mxfdv-dif.c: (mxf_dv_dif_handle_essence_element):
* gst/mxf/mxfjpeg2000.c: (mxf_jpeg2000_handle_essence_element):
* gst/mxf/mxfmetadata.c: (mxf_metadata_sequence_init),
(mxf_metadata_structural_component_init),
(mxf_metadata_generic_picture_essence_descriptor_init):
* gst/mxf/mxfmpeg.c: (mxf_mpeg_video_handle_essence_element),
(mxf_mpeg_audio_handle_essence_element):
* gst/mxf/mxfparse.h:
* gst/mxf/mxfup.c: (mxf_up_handle_essence_element):
* gst/mxf/mxfvc3.c: (mxf_vc3_handle_essence_element):
* tests/check/elements/mxfdemux.c: (_sink_chain):
Implement support for OP2a/b/c and OP3a/b/c, i.e. tracks with
more than a single component. This currently only works for
the case where the components are stored in playback order
in the file.
Set some more default/distinguished values for the structural
metadata.
Make some types more strict by choosing the correct subclasses.
Set DISCONT flag on buffers after a component switch.
Take the last partition from the random index pack for the footer
partition of the header partition doesn't reference the footer
partition. This gives us the final structural metadata for
some more files in the beginning.