matroskademux: support for multi-segment Matroska files

Submitted by Lionel Dricot @ldricot

Link to original bug (#334082)

Description

If you play a matroska (.mkv) file with segments, Totem will only play the
first segment without allowing you to switch to another one.

Unfortunatly, I can't upload an example file (I've seen this with a 4Go file).
Perhaps is there one in the Gstreamer test suite ?

Following Haali on #matroska, it's not about chapter but about segment.

Most files are one segment only and totem only support one segment (as all
others *NIX players).

My file is multi-segment, which is one step over chapters. (If I understand
correctly the spec, each segment can have his own tracks and chapters. Chapters
are only "bookmarks" in a big file)

The spec : http://www.matroska.org/technical/specs/index.html