Add qtSMTKCallObserversOnMainThreadBehavior, for vtk-only apps.
A parallel to pqSMTKCallObserversOnMainThreadBehavior for vtk-only apps, so observation observers get called on the main thread.
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A parallel to pqSMTKCallObserversOnMainThreadBehavior for vtk-only apps, so observation observers get called on the main thread.