No longer call OnDeserialize() when deserializing a scene with Scene::Deserialize().

This doesn't really make that much sense. The main purpose of OnDeserialize() is for
things like save games and moving data between scene transitions. The next commit will
be disabling OnSerialize() when Scene::Serialize() is called, for the same reason.

Issue #88.
No longer call script-side OnSerialize() in Scene::Serialize().

This relates to the previous commit. Doesn't make much sense in this context.

There is still an issue where it will be called in the scene state stack.

Issue #88.
Fix a bug when "intelligently" placing an object on the X/Z plane.
Fix a bug where peeking at a scene node's ID from serialized data would return an incorrect result.

Fixes issue #97.