introspect: implement enough to make gnome-control-center work

We can see sources and sink and change the volume/mute of streams

src/internal.h

@@ -336,6 +336,7 @@ struct pa_stream {
 	uint32_t buffer_offset;
 
 	float volume;
+	bool mute;
 	pa_operation *drain;
 	uint64_t queued;
 };