Implement an argument-hint widget similar to the completion-widget that is attached to the completion-widget and gets it's data through the completion-models. Extract the expandable-item logic from KateCompletionModel into a more abstract model, and use that as base-class for KateCompletionModel and the new KateArgumentHintModel.

Add the roles into CodeCompletionInterface that will be needed to selectively match completion-items to argument-hints, and the roles needed to determine the argument-hint depth.