Add parent_id

Add parent_id to globals to make hierarchy of interfaces. We can use
this to group interfaces or to describe the owner.

src/pipewire/node.h

@@ -148,6 +148,7 @@ struct pw_node {
 struct pw_node *
 pw_node_new(struct pw_core *core,		/**< the core */
 	    struct pw_resource *owner,		/**< optional owner */
+	    struct pw_global *parent,		/**< optional parent */
 	    const char *name,			/**< node name */
 	    struct pw_properties *properties,	/**< extra properties */
 	    size_t user_data_size		/**< user data size */);