Integrate Collaboration features into the online framework

[istvanrath]

• make sure the online framework maintains an in-memory EMF-compliant (EObject-oriented) representation of the model(s) in the online session
• apply the "lens concept" and derive a filtered model for each participating user (i.e. multiple front models per session)
• make sure notation elements (sizes, positions etc) are part of the EMF model and are subject to lens rules