gst/matroska/matroska-demux.c: Fix duration query; fix basetime in newsegment event after seek; fix duration in initi...

Original commit message from CVS:
* gst/matroska/matroska-demux.c:
(gst_matroska_demux_handle_src_query),
(gst_matroska_demux_handle_seek_event),
(gst_matroska_demux_loop_stream_parse_id):
Fix duration query; fix basetime in newsegment event after
seek; fix duration in initial newsegment event.
* gst/matroska/matroska-mux.c:
(gst_matroska_mux_audio_pad_setcaps), (gst_matroska_mux_start):
Extract number of channels and samplerate from vorbis headers;
add some debug messages when querying the durations of the
input streams.