Issue #2004244 by Berdir, plach, tim.plunkett, msonnabaum, das-peter: Move entity revision, content translation, validation and field methods to ContentEntityInterface.

Issue #1810370 by plach, kfritsche, berdir, alexpott, xjm, chx: Entity Translation API improvements.

Issue #1620010 by dawehner, ParisLiakos, plach, jibran: Move LANGUAGE constants to the Language class.

Issue #1446382 by plach: Need a reliable way to determine if a specific bundle for an entity type is translatable.

Issue #1877632 by fago, jhodgdon: Improve comments and clean-up code for EntityNG and the TypedData API.

Issue #1696640 by fago, effulgentsia, tim.plunkett, dixon_, plach: Add a uniform Entity Property API.