Merge pull request #2651 from ace20022/lang_codes

Fix and extend ISO 639 lists
FIX: Allow "Scan for new content" on folders
Merge pull request #2645 from koying/fixscanfolder

FIX: Allow "Scan for new content" on folders
[Settings] Add m_subtitleExtensions.
Find external audio files, open corresponding inputStreams and store them.
Adapt all relevant places where m_pDemuxer is used.
In particular ReadPacket(...) and add ReadExternalAudioPacket(...),
open demuxers for external audio files,
update the SelectionStream.
Adapt DVDFileInfo so that it reads infos also from external audio files (e.g. language and channels).
[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(...).
[Util] Remove obsolete method ScanArchiveForSubtitles() and sub_exts[].
[Util] Adapt ScanForExternalAudio(...) to use the generalized methods.
ScanForExternalAudio(...) now uses GetPathsToLookForAssociatedItems() and ScanPathsForAssociatedItems(...). ScanArchiveForAudio(...) is removed.
[Util] Add Doxygen comment for ScanForExternalAudio(...)
[Util] Cosmetic: rename nAudioAdded to nItemsAdded.
[DVDPlayer] Cosmetics part 1.
[DVDPlayer] Simplify ReadPacket(...).
[DVDPlayer] Test elupus' idea.