[Ext. Subtitles][Redux] Add subtitles language and name extraction from filename and VideoPlayer.SubtitlesLanguage infolabel

With @pieh's blessing I took over his PR#1365. I rebased it but haven't touched the commits' author info.
It would be nice if the feature could make it into the next merge window.

I have also worked on an solution to store these info in the db: https://github.com/ace20022/xbmc/commit/48388e8f47e68725d59298d7c000e5b3f4040380, but that only works upon play or when the thumbnail extractor runs.
@jmarshallnz are there any plans to update the db automatically? While idle or from time to time or something like that?
Storing the info in the db can be useful, e.g., for language flags: http://forum.xbmc.org/showthread.php?tid=155693.