Re indent code and add member to know if the value description is animated value node.
Move the get/set members for interpolation property to Value Node class. Add custom interface for derived classes that can implement it.
Simplify checking of given value description to allow to show the 'Set Interpolation' contextual menu
Simplify code to perform or undo the ValueDescSetInterpolation action based on if the value description is a value node or not.