matroskademux: Refactor track parsing out from adding tracks

This splits gst_matroska_demux_add_stream() into:

* gst_matroska_demux_parse_stream(): will read the Matroska bytestream
  and fill a GstMatroskaTrackContext.

* gst_matroska_demux_parse_tracks(): will check there are no repeated
  tracks.

* gst_matroska_demux_add_stream(): creates and sets up the pad for the
  track.

https://bugzilla.gnome.org/show_bug.cgi?id=793333