gst/matroska/: Use a GArray for storing the Cue (i.e. seek) information, store the CueTrackPositions for every track,...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_reset),
(gst_matroskademux_do_index_seek),
(gst_matroska_demux_parse_index_cuetrack),
(gst_matroska_demux_parse_index_pointentry),
(gst_matroska_index_compare), (gst_matroska_demux_parse_index),
(gst_matroska_demux_parse_metadata):
* gst/matroska/matroska-demux.h:
* gst/matroska/matroska-ids.h:
Use a GArray for storing the Cue (i.e. seek) information, store
the CueTrackPositions for every track, store the block number
and optimize searching in the array by sorting it after the last
element was added.
Fix a small memory leak when trying to parse a tags element that was
already parsed.