Separated code completion queries into CodeCompletionServices

This pull request separates code completion queries into a `CodeCompletionServices` class and out of the `CodeBlockEditor` UI component so that it can be tested independent of the UI, `DynamoModel` etc. with its own core and own set of libraries. This is part one of a series of submissions involving this change. The PR's that will follow will include calls into `CodeCompletionServices` from the UI as well as rewritten unit tests making use of only `CodeCompletionServices`.

@Benglin please review.
Just as we instantiate it here, I would recommend setting it to ```null``` in ```EngineController.Dispose``` method (this way if some code happens to access it after ```EngineController``` is disposed of, then things will crash and we'll find out problems in a more visible way.