Introduce validation method and class in XBlock API

@cpennington and @dmitchell please review

You can see the Studio extension of the Validation class in common/lib/xmodule/xmodule/validation.py in https://github.com/edx/edx-platform/pull/5710 (which is still WIP so is not yet commented or tested).
I'm surprised to see you storing the actual messages as just a dictionary. I thought you wanted Studio to be able to provide messages with more information?