Add support for downloading slides about bbb-downloader

[olberger]

The bbb.py script parses the shapes.svg file, in order to extract the URLs of the slides. It's just a matter of adding support for download of these... PR welcome ;)

[olberger]

Oh, and no real need to check youtube-dl, since I've copied the bbb.py script from there to here ;-)

[olberger]

The slides are downloaded now by download_bbb_data.py, but they aren't integrated in the videos directly downloaded, unless you use capture-full-replay.sh

[olberger]

However, it may be possible to parse the shapes.svg, to extract duration of each slide's presentation, and to integrate all of these in a video sequence with https://trac.ffmpeg.org/wiki/Slideshow#Concatdemuxer
If anyone volunteers to propose a PR, it will be much welcome.