playlist_queue: resolve relative URIs, database lookup

Prepend the playlist's base URI to relative song URIs.  Look up songs
in the database (if the URI refers to a local song file).  Merge
existing database metadata with metadata from the playlist plugin.

src/playlist_queue.h

@@ -32,9 +32,12 @@ struct playlist;
 /**
  * Loads the contents of a playlist and append it to the specified
  * play queue.
+ *
+ * @param uri the URI of the playlist, used to resolve relative song
+ * URIs
  */
 enum playlist_result
-playlist_load_into_queue(struct playlist_provider *source,
+playlist_load_into_queue(const char *uri, struct playlist_provider *source,
 			 struct playlist *dest);
 
 /**