playbin/decodebin: Add support for closed captions extraction from metas

I found some sample video files [1] with CEA-608 closed captions that are not recognized by gstreamer (tested with the git version from some days ago [2]). 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".

[1] https://drive.google.com/drive/folders/0B_x8BEhWfD7yNS1RWnpYMjJsczA

[2] 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