Find external audio files, open corresponding inputStreams and store them.
Open demuxers for external audio files. Update the SelectionStream. Adapt all relevant places where m_pDemuxer is used; in particular ReadPacket(...) and add ReadExternalAudioPacket(...)
Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).
Only apply Seek and SetSpeed to the actually active external demuxer to speed things up.
Fix seeking chapters while having an ext. audio track selected.
(External) audio tracks don't have chapters, so we have to use SeekTime().