Auto-complete function signatures - Part 2

@Benglin - I have split up PR: #2688 into smaller PR's, of which this is the second. This is independent of the first: https://github.com/DynamoDS/Dynamo/pull/2735 but still better to review them in order.
Please remove the setter for ```ClassNode``` property if it is not used. If it is, then we need to understand what type of scenario would we be required to set ```ClassMirror.ClassNode``` property from an external code, because that sounds wrong to me.