Restore function evaluation feature in SimpleExpression view
The function evaluation feature in SimpleExpression view relies on VTK. However, also do we want to be able to preview this view with only Qt enabled. In order to do so this MR creates a subclass of qtSimpleExpressionView and it would override the SimpleExpression view constructor at runtime. That being said now users can preview it with only Qt enabled and do function evaluation when both Qt and VTK are enabled.
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