[PATCH] upnp: add support for thumbnails

[Guillaume Emont <gemont igalia com>]

Ok, this is a new version, that supports upnp:albumArtURI and set things up
properly in _supported_keys().

In the end, I did not implement PNG_* nor JPEG_SM, as they seem relatively
uncommon to me. They can easily be added later on though.

And since thumbnails returned by grilo can be small, I think it would be a bad
idea to expect them to be of good enough quality for use as background, in a
cover flow animation or anything like that.

If we want to support that kind of thing, I think we would need either another
value, or a multi-valued property like Juan suggested (but then I think it
should provide the resolution of the image, or at least a resolution range that
the application can expect).


Guij