Issue #3420986 by kim.pepper, sorlov, larowlan, mstrelan: Convert Condition plugin discovery to attributes

Revert "Issue #2852463 by mikelutz, tim.plunkett, heddn, xjm, MegaChriz, joachim, phenaproxima, dawehner, benjifisher: Create a trait to implement \Drupal\Component\Plugin\ConfigurableInterface"

This reverts commit 109dfd58.

Issue #2852463 by mikelutz, tim.plunkett, heddn, xjm, MegaChriz, joachim, phenaproxima, dawehner, benjifisher: Create a trait to implement \Drupal\Component\Plugin\ConfigurableInterface

Issue #2776975 by joelpittet, dawehner, tim.plunkett, xjm, pfrenssen: March 3, 2017: Convert core to array syntax coding standards for Drupal 8.3.x RC phase

Issue #2537732 by Xano, tim.plunkett, gnuget, claudiu.cristea, drunken monkey, bojanz: PluginFormInterface must have access to the complete $form_state (introduce SubFormState for embedded forms)

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

Issue #2375695 by Berdir, EclipseGc, tim.plunkett, Wim Leers, Fabianx, dawehner: Condition plugins should provide cache contexts AND cacheability metadata needs to be exposed

Issue #2339151 by EclipseGc, tim.plunkett, Gábor Hojtsy, effulgentsia: Conditions / context system does not allow for multiple configurable contexts, eg. language types

Issue #1182850 by xjm, Stefan Lehmann, jmcintyre, Bojhan: Remove full stops at the end of checkbox options.

Issue #2305569 by fago: Fixed Follow-up to context definitions: Remove unused, left-over static contextDefinitions() methods.

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

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

Issue #1986802 by yched: Rename PluginInspectionInterface::getDefinition() to getPluginDefinition().