server: Set the album art uri when applying didl
server: Set the album art uri when applying didl

Currently if an object is created in a writable container the
album art uri is thrown away.
This commit makes sure to create a thumbnail in the music item
and assign the album art URI from the DIDL, if one is provided.

Signed-off-by: Richard Röjfors <richard@puffinpack.se>

https://bugzilla.gnome.org/show_bug.cgi?id=757056

GNOME/rygel,GNOME/rygel,GNOME/rygel