[Util] Generalize external subtitle retrieval method(s).

Add void CUtil::GetPathsToLookForAssociatedItems()
Add void CUtil::ScanPathsForAssociatedItems(...)
Add int CUtil::ScanArchiveForAssociatedItems()
[Util] Make use of generalized ScanForExternalSubtitles(...) methods.
ScanForExternalSubtitles(...) now uses GetPathsToLookForAssociatedItems() and ScanPathsForAssociatedItems(...).
Remove obsolete method ScanArchiveForSubtitles() and sub_exts[].
[Util] Add method to scan for external audio files.
[DVDPlayer] Open input streams for internal and ext. audio files and store them.
[DVDPlayer] Add functionality to 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(...)
[DVDPlayer] Handle bogus/erroneous ext. audio files.
[DVDFileInfo] Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).