Feature: Optional validator for all Actions

[kafonek]

I want something I can add to all Actions to validate the attached handlers. Some of this functionality would overlap with potentially adding .default_handlers, I'm not sure where the boundaries would be there. The Validator would take the Action as its main argument. For instance:

• Make sure every Action has a specific handler like Status handling attached to it
• Watch for unhandled messages across multiple handlers -- e.g. is update_display_data not handled by any of the attached handlers