* Enhance the API Scanner plugin to monitor class annotations.
* Implement @DoNotImplement annotation, and apply it.
* Update API definition.
* Update API change detection to handle @DoNotImplement.
* Document the `@DoNotImplement` annotation.
* Scan API for addition of new abstract methods.
* Make sure we ignore blank lines when counting API changes.
* Add 6 new abstract APIs to our API definition.