HLS demux proof of concept:

 - exposes renditions as pads
 - does not queue complete fragments, so it starts playing very quickly

A lot of features are missing, the most important ones being proper playlist
update scheduling, automatic stream switching based on the download rate. Also
right now _all_ renditions will be downloaded when the demuxer is used in a
decodebin, since there is no stream selection API.

The gsturidownloader.[ch] files are modified versions of the ones in
gst-plugins-bad.