Rygel and DMP support

[Milan Plzik <milan plzik streamunlimited com>]

Good day,

  company I work for considers to use Rygel/gupnp/... as a UPnP stack 
in one of its products.  After some basic analysis and testing, we found 
out some limitations we would like to discuss:

   1) RygelMediaPlayer and playlist/multi-track support


   RygelMediaPlayer's interface does not provide API (nor any other
   public clas does) making playlist handling possible (e.g.
   next/previous commands, and also reporting of both current media and
   track).

   Since Rygel handles playlist logic internally in
   RygelPlayerController, it can lead to cases where next/prev controls
   don't work (if media playback was initiated via rygel, local UI does
   not know about media and vice versa).

   Without breaking ABI, this might be solvable by making
   RygelPlayerController public and enabling user to re-implement it,
   but it would be good to discuss this a bit more to have merge-able fix.


   2) Error reporting

   Rygel's API is not designed to allow error reporting (error codes)
   from player to UPnP control point in first place; e.g. there is no
   way to report an error client when invoking play() while no media is
   set. Rygel's code currently does not handle these cases and there is
   no hint about whose responsibility is to return error code. If it is
   solely rygel's responsibility, we will gladly supply patches which
   fix this behavior, otherwise the API might need significant changes.

Currently, these two issues are main blockers for us and we are 
interested in fixing them as soon as possible.If you have any 
suggestions/comments to any of these issues, we would like to merge our 
fixes to the upstream.

    Milan Plzik