In Scene.AddNewAgent(), avoid a situation where an exception can result in a client being added to the manager without IClientAPI.SceneAgent being set.

This is done by adjusting the order of code so that SceneAgent will always be set before adding the client.
Various parts of the code (rightly) assume that a a client registered to the manager will always have a SceneAgent set no matter what.
minor: Correct letter accidentally added to copyright notice on top of Scene.cs (almost five years ago!)