gst: use target.object instead of node.target, soft-deprecate ids

Use target-object=<serial/name> instead of path=<id> for specifying
sink/src targets. Deprecate path= argument.

Change device provider to preferably expose serials instead of ids.

src/gst/gstpipewiredeviceprovider.h

@@ -56,6 +56,7 @@ struct _GstPipeWireDevice {
 
   GstPipeWireDeviceType  type;
   uint32_t            id;
+  uint64_t            serial;
   const gchar        *element;
 };