Issue #1761040 by sun, tim.plunkett, fago, effulgentsia: Rename Storable, Entity, and Configurable to Entity, ContentEntity, and ConfigEntity.

Issue #1668820 follow-up by tim.plunkett, sun: Documentation fixes and clean-ups to Configurables patch.

Issue #1668820 by sun, xjm, tim.plunkett, fgm: Concept, base class, and interface for configurable objects.

Issue #1609760 by chx, sun: Fixed hook_image_style_()*() is not invoked for image styles upon Image module installation.