policy-node: improve target device selection logic

-loop through all the linkable nodes, instead of using default nodes
-implement a priority scheme to pick the appropriate target linkables
-If both nodes support a passthrough format, check the intersection.
 If the intersection is empty and one of the node is passthrough only,
 skip the node as a target
-the logic now closely aligns with the media-session logic

Fixes #75

src/scripts/create-item.lua

@@ -21,6 +21,7 @@ function configProperties(node)
     ["node.id"] = node["bound-id"],
     ["client.id"] = np["client.id"],
     ["object.path"] = np["object.path"],
+    ["priority.session"] = np["priority.session"],
   }
 
   for k, v in pairs(np) do