Move event types constants into single file

Move all constants for event types to types.go for easier code
readability and maintainance.

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

supervisor/add_process.go

@@ -35,7 +35,7 @@ func (s *Supervisor) addProcess(t *AddProcessTask) error {
 	t.StartResponse <- StartResponse{}
 	s.notifySubscribers(Event{
 		Timestamp: time.Now(),
-		Type:      "start-process",
+		Type:      StateStartProcess,
 		PID:       t.PID,
 		ID:        t.ID,
 	})