[2113] Fix control nodes migration for existing GV/AFV diagrams

[jerome-obeo]

Due to changes introduced to support graphical control nodes on the AFV diagram background, opening existing GV and AFV diagrams containing control nodes caused those nodes to be rendered both on the diagram background and within the action flow compartment.

This commit fixes this behavior by hiding the extra top-level control nodes, which should remain hidden.

Bug:#2113

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