[2113] Handle control node graphical nodes on AFV diagram background

[jerome-obeo]

Bug:#2113

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[AxelRICHARD]

Why do you want to change that?
Is there a problem with adding them as children of the FakeNodeDescription?

[jerome-obeo]

Since there is no reusedChildren api on DiagramDescription, I don't see other option than moving all node descriptions as children of diagram description.