node: add group_id property

Nodes with the same group id are scheduled by the same driver, even
if they are not in any way linked to eachother.

src/pipewire/private.h

@@ -536,6 +536,7 @@ struct pw_impl_node {
 	char *name;				/** for debug */
 
 	uint32_t priority_driver;	/** priority for being driver */
+	uint32_t group_id;		/** group to schedule this node in */
 	uint32_t spa_flags;
 
 	unsigned int registered:1;