Feature suggestion: also clear scene events when scene shutdown

[rexrainbow]

When scene shutdown

• All game objects will be destroyed by engine,
  • Registered events on game objects will also be clear, too.
• Touch, keyboard and GamePad events will be clear by engine,
• Cameras will be destroyed by engine

But scene events (e.x. scene.events.on('update', callback)) won't be clear by engine. User has to remove registered events manually.

Add events.removeAllListeners() at the end of Systems.shutdown() method might clear scene events.