Imports for field arguments are not added

[kito99]

Given the following schema:

type Query {
  widgets(
    first: Int
    after: String
    colors: [String]
  ): WidgetResponse
}

The widgets() method is correctly generated, however the import java.util.List is not added to the Query.java file. A work-around for this is to modify the template (graphql-apigen.stg) to always add java.util.List, but, this is hardly ideal :-). I think this could be solved by modifying STModel.getImports() to look at arguments of fields.