gst/matroska/: Parse Attachments and post them as GST_TAG_IMAGE if we detect it as image and otherwise as GST_TAG_ATT...

Original commit message from CVS:
* gst/matroska/Makefile.am:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_reset),
(gst_matroska_demux_parse_attached_file),
(gst_matroska_demux_parse_attachments),
(gst_matroska_demux_parse_contents_seekentry),
(gst_matroska_demux_loop_stream_parse_id):
* gst/matroska/matroska-demux.h:
* gst/matroska/matroska-ids.c: (gst_matroska_register_tags):
* gst/matroska/matroska-ids.h:
* gst/matroska/matroska.c: (plugin_init):
Parse Attachments and post them as GST_TAG_IMAGE if we detect
it as image and otherwise as GST_TAG_ATTACHMENT. Include filename
and description of the attachments in the caps. Fixes bug #537622.