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.
Surprised that you didn't change this to ```IEnumerable<KeyValuePair<string, string> ArgumentList```?