Generalize component/AI serialization system to any user-defined JS
object. This includes Vector2D/3D, fixing an old issue.
As with components/AI, JS Objects may implement a Serialize/Deserialize
function to store custom data instead of the default, which attemps to
serialize all enumerable properties.
Fixes#4698
Differential Revision: https://code.wildfiregames.com/D2746
This was SVN commit r24462.