event_pipe: added pipe_event enum and callbacks

Make the event_pipe (formerly main_notify) send/receive a set of
events, with a callback for each one.

The default event PIPE_EVENT_SIGNAL does not have a callback.  It
is still there for waking up the main thread, when it is waiting for
the player thread.

src/idle.c

@@ -66,7 +66,7 @@ idle_add(unsigned flags)
 	idle_flags |= flags;
 	g_mutex_unlock(idle_mutex);
 
-	event_pipe_signal();
+	event_pipe_emit(PIPE_EVENT_IDLE);
 }
 
 unsigned