Issue #3420984 by godotislate, andypost, smustgrave, larowlan, alexpott: Convert Layout DisplayVariant, PageDisplayVariant discovery to attributes

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2318281 by tim.plunkett, dawehner | kim.pepper: Provide a standard way to check a plugin is an instance of the required interface.

Issue #2006278 by Wim Leers: Remove leftover 'module' annotation keys and plugin manager processDefinition() to deal with disabled modules during tests.

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

Issue #1930020 by dawehner, Berdir, olli: Inject a class into the Core annotated discovery implementation so that namespaces can be updated.

Issue #1836008 by dawehner, Berdir, alexpott, katbailey, effulgentsia, EclipseGc: Fixed Remove drupal_classloader() use in Drupal\Core\AnnotatedClassDiscovery.

Issue #1874936 by frega, Wim Leers: Pluggable in-place editors (allow modules to define new Create.js PropertyEditor widgets).

Issue #1824500 by Wim Leers, tkoleary, frega, jessebeach, henribergius, effulgentsia, nod_, yched: In-place editing for Fields.