Issue #1927608 by EclipseGc, tim.plunkett, effulgentsia: Remove the tight coupling between Block Plugins and Block Entities.

Issue #1987298 by tim.plunkett, quicksketch: Shorten directory structure and PSR-0 namespacing for plugins.

Issue #1982024 by Cottser, Fabianx: Lazy-load Attribute objects later in the rendering process only if needed.

Issue #1947814 by vijaycs85, Gábor Hojtsy, tim.plunkett: Fixed New config entities often save as langcode: und incorrectly.

Issue #1875260 by Gábor Hojtsy, xjm, brantwynn, EclipseGc, Bojhan: Fixed Make the block title required and allow it to be hidden.

Issue #1935000 by Gábor Hojtsy, vijaycs85: Fixed Some configuration entities are created as in language unknown.

Issue #1591806 by tim.plunkett, ryanissamson, jenlampton, Risse, psynaptic, Gábor Hojtsy, kmcculloch, pakmanlh, Rob Loach: Change block 'subject' so that it's called (admin_)label like everything else on the theme layer.

Issue #1891516 by sun, Berdir, chx: Remove $install parameter from DrupalUnitTestBase::enableModules(), encourage to create individual schema tables only.

Issue #1871696 by tim.plunkett, xjm, sun, msonnabaum, EclipseGc: Convert block instances to configuration entities to resolve architectural issues.