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).
The StudioValidation class does add more fields to the dict. See StudioValidation (xmodule/validation.py) in edx/edx-platform#5710.

A dict turned out just to be the easiest thing.