gst/mxf/: Add property to select the top-level source package or material package...

gst/mxf/: Add property to select the top-level source package or material package that should be used for playback.

Original commit message from CVS:
* gst/mxf/mxfdemux.c: (gst_mxf_demux_remove_pads),
(gst_mxf_demux_reset), (gst_mxf_demux_find_package),
(gst_mxf_demux_choose_package),
(gst_mxf_demux_handle_header_metadata_update_streams),
(gst_mxf_demux_handle_klv_packet), (gst_mxf_demux_set_property),
(gst_mxf_demux_get_property), (gst_mxf_demux_finalize),
(gst_mxf_demux_class_init):
* gst/mxf/mxfdemux.h:
* gst/mxf/mxfparse.c: (mxf_umid_from_string):
* gst/mxf/mxfparse.h:
Add property to select the top-level source package or material
package that should be used for playback.