Duplicating projects/scenes produces events with unexpected `update_base` change type

[YanehCheck]

While no functionality is directly impacted by this, using update_base instead of the add change type is, to say the least, unexpected. I think this decision stems from the add change type not being used anywhere else for scenes and projects (as new scenes/projects are de-facto notified by the OpenScene/OpenProject events... and are not really saved on the server side until a client invokes SaveScene/SaveProject RPC).

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As this is a breaking change for the API, these would be the related modifications for C# client libraries:

1. Communication library - Arcor2Client.cs (lines: 844)):
   Define and invoke a new SceneAdded event on the right change type of the SceneChanged event message.
2. ClientServices library - Arcor2Session.cs (lines: 747, 892)):
   Set up registration, unregistration, and logic for the new SceneAdded event. This event will just add a new scene, similar to the old SceneBaseUpdate, except it doesn't need to check for the non-existence of the scene (e.g., to recognize duplicate x rename for existing). The SceneBaseUpdate event logic can be marked and left intact for compatibility.