mkv: added matroska_segment_seeker.{hpp, cpp}

The relevant files has been added to the Makefile.am in charge of the
demuxers.

The code added will be responsible for efficient seeking in mkv files.
It might look very complex but the theory behind it is quite simple:

    - Use the cues in the files as guiding to find where we should look
      for proper index points; the indexes we actually find will be used
      as seekpoints.

    - Keep track of every section we have "indexed", meaning that if we
      jump to a location where we already know where the proper
      seekpoints are; do not go through it one more time.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>