Created Task: Designer: Downgrade model-first models when ef6 selected on net40 [1338]

For empty models (a.k.a. model-first) we create a v3 edmx on projects that target .NET Framework 4. When the user first tries to generate a database from the model or update the model from a database, the have to select an EF runtime version. If the user selects EF5, we will need to downgrade the edmx to v2.

If v3-only features are used in the model, we should also consider blocking the user from selecting EF5 or warning if they do.