Evaluates the given {@link IJSExpression} as a constant expression and returns the result. At the moment this only supports
	 * very simple expressions involving unary operators and literals alone. Support for more complex expressions will be added as
	 * needed.
Evaluates the given {@link IJSExpression} as a constant expression and returns the result. At the moment this only supports
	 * very simple expressions involving unary operators and literals alone. Support for more complex expressions will be added as
	 * needed. | QMLExpressionEvaluator.java