cubicweb-mediaplayer #9216988 Use JSON output of avprobe [deprecated]

Instead of parsing the output of avprobe with regular expressions, recent versions have a JSON output with a command like:

avprobe tmp/video.flv -of json -show_format -show_streams

(tested on Debian Jessie with avprobe version 11.4-6:11.4-1~deb8u1)

prioritynormal
typeenhancement
done in<not specified>
load0.000
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