gst/matroska/matroska-demux.c: Handle position and duration query in DEFAULT format if the pad's track has a default ...

Original commit message from CVS:
* gst/matroska/matroska-demux.c: (gst_matroska_demux_class_init),
(gst_matroska_demux_add_stream), (gst_matroska_demux_query),
(gst_matroska_demux_element_query),
(gst_matroska_demux_handle_src_query),
(gst_matroska_demux_handle_seek_event):
Handle position and duration query in DEFAULT format if the
pad's track has a default frame duration set.
Fix seeking now that the segment's duration doesn't contain the
(possibly wrong or inaccurate) duration of the Matroska file.