Fix missing teletext subtitles

This includes https://github.com/opdenkamp/xbmc-pvr-addons/pull/254

It fixes a long-time regression which caused some teletext subtitles (at least from tvheadend) to not show up. The problem was two-fold: the code incorrectly ignored TEXTSUB streams (I believe those are teletext streams that tvheadend decodes into raw text streams), and the stream parser incorrectly stored the composition and ancillary ID for all types of subtitles when it should only have been doing it for DVBSUB type streams. This last part is only fixed for pvr.hts so far, I'll push a fix for pvr.vdr.vnsi soon as well.

Reference: http://forum.xbmc.org/showthread.php?tid=169386

@opdenkamp and @FernetMenta please review.