Filter classes in code completion by HiddenInLibrary class attribute that hides them in library

@Benglin 
Here's a suggestion on how this class can be rewritten:

```cs
public class SampleClassA
{
}

[IsVisibleInDynamoLibrary(false)]
public class SampleClassB
{
}

public class SampleClassC
{
}
```

Ensure that only ```SampleClassA``` and ```SampleClassC``` show up in commpletion list.