playbin/decodebin: Add support for closed captions extraction from metas
I found some sample video files  with CEA-608 closed captions that are not recognized by gstreamer (tested with the git version from some days ago ). Those files play nicely on mpv.
For example, I tried running
gst-play-1.0 1021_20150103051932.mpg and then pressing
s a few times, which outputs the message "No subtitle tracks".
 The exact versions used are: (date and commit at the end)
gstreamer-1.15.1 20190205 febfabb39 gst-plugins-base-1.15.1 20190205 91863b071 gst-plugins-good-1.15.1 20190206 062f2c46f gst-plugins-bad-1.15.1 20190206 fffb2aa12 gst-plugins-ugly-1.15.1 20190130 de65f886