mkv soft subtitle support

mkv soft subtitle support
- new libmkv 0.6.4 with subtitle track support
- muxmkv supports vobsub and closed caption subtitles
- added subtitle format, source, and dest initialization to dvdnav
- moved subtitle_force flag into hb_subtitle_t struct as it needs to
  be settable per subtitle
- gtk ui added subtitle tab which allows selection of multiple subtitles
- reorgainize subtitle sync code to prevent dropping of subtitles when
  multiple subtitles are enabled

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@2428 b64f7644-9d1e-0410-96f1-a4d463321fa5