When switching scenes, support optionally saving the state

[moolicc]

When switching scenes, we should allow the user to optionally save the current scene/restore the incoming scene. We should thus support three switching modes:

• Retain
  • Scene is retained in memory as-is. Only change is that update/draw events no longer fire.
• Save to file
  • Scene should be serialized to json and saved to a file.
• Save in memory
  • Scene should be serialized to json but kept in memory.