playback: Support for VobSub subtitles (.idx files)

Submitted by Sven Arvidsson @whizse

Link to original bug (#583267)

Description

With bug 350311 resolved, GStreamer should now support subpicture subtitles, but it doesn't seem to work in Totem.

All I get when I launch video.avi with a video.sub as a subtitle is this:

** Message: Error: Internal data stream error.
gstmpegdemux.c(2536): gst_flups_demux_loop (): /GstPlayBin2:play/GstURIDecodeBin:uridecodebin1/GstDecodeBin2:decodebin20/GstMpegPSDemux:mpegpsdemux0:
No valid streams detected

This might even be a regression, as previous versions just ignored the .sub file (if it wasn't a text subtitle) and let me play the video.