Adds a basic support for local actor references in Prefabs.

[OmikronX]

I have added basic support for local actor references.
I use ItemID to lookup the correct actor when restoring the actor state.
It is way from perfect/finished and I don't think you want to pull it as is.
However, I hope it will give you a starting point to improve upon.

The known limitations are:

1. it does not support nested prefabs
2. soft references are kept unchanged - but I consider this feature as it allows you to reference the same actor from all prefabs while hard references are prefab specific