[vtkusers] Make trackball interactor style default

[John Wilson]

I'd like to have the default style for vtkRenderWindowInteractor set at
trackball.  I've tried to do this with the following:

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vtkInteractorStyleTrackball interStyle

vtkRenderWindowInteractor iren
    iren SetRenderWindow renWin
    iren SetUserMethod {wm deiconify .vtkInteract}
    iren SetPicker picker
    iren LightFollowCameraOn
    iren SetInteractorStyle interStyle
    iren Initialize

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But this doesn't work.  Any insight?

Thanks -

John