Issue #2384481 by rpayanm, daffie, tadityar, hussainweb, areke, claudiu.cristea: Make the class variables protected for NodeType

Issue #2281635 by fubhy, er.pushpinderrana, EclipseGc, blueminds, tim.plunkett: \Drupal\Core\Plugin\Context\Context should use a ContextDefinition class instead of arrays.

Issue #2278541 by tim.plunkett, kim.pepper, Bojhan, EclipseGc: Refactor block visibility to use condition plugins.

Issue #2283929 by tim.plunkett: Clean up condition plugins to have schema support and to allow them to be optional.

Issue #2272161 by tim.plunkett, fago: Entity-based conditions should use type => entity: instead of constraints.

Issue #2074509 by tim.plunkett, falcon03, mgifford, Xano: Fixed Add missing #title property to ensure form accessibility.

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

Issue #1849752 by effulgentsia, Berdir, jibran, neclimdul: Abstract non-Drupal-specific parts of AnnotatedClassDiscovery into a Drupal\Component base class.