[DVDPlayer] Open input streams for internal and ext. audio files and store them.
[DVDPlayer] Add open external demuxers and read from them.
Update the SelectionStream.
Adapt all relevant places where m_pDemuxer is used; in particular ReadPacket(...) and add ReadExternalAudioPacket(...)
[DVDFileInfo] Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).
[DVDPlayer] Handle bogus/erroneous ext. audio files.