[GREENFOOT-645] Add the new-scenario action (incl wizard) to JavaFX interface

The new scenario action, including its wizard, needs to be moved across to the JavaFX interface.  I suspect its implementation will become a lot simpler, because it will no longer need to cross the VM boundary and pass that wizard parameter when making the debug VM.  All that old code should be removed as part of this ticket.

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Issue metadata

• Issue type: Task
• Priority: Medium