Issue #1743686 by EclipseGc, fago, amateescu, damiankloip: Added Condition Plugin System.

core/lib/Drupal/Component/Plugin/Context/Context.php

.../lib/Drupal/Component/Plugin/Context/ContextInterface.php

core/lib/Drupal/Component/Plugin/ContextAwarePluginBase.php

...b/Drupal/Component/Plugin/ContextAwarePluginInterface.php

core/lib/Drupal/Core/Condition/ConditionInterface.php

core/lib/Drupal/Core/Condition/ConditionManager.php

core/lib/Drupal/Core/Condition/ConditionPluginBase.php

core/lib/Drupal/Core/CoreBundle.php

core/lib/Drupal/Core/Executable/ExecutableException.php

core/lib/Drupal/Core/Executable/ExecutableInterface.php

...lib/Drupal/Core/Executable/ExecutableManagerInterface.php

core/lib/Drupal/Core/Executable/ExecutablePluginBase.php

core/lib/Drupal/Core/Plugin/Context/Context.php

core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php

...n/Validation/Constraint/EntityTypeConstraintValidator.php

...s/node/lib/Drupal/node/Plugin/Core/Condition/NodeType.php

...ode/lib/Drupal/node/Tests/Condition/NodeConditionTest.php

...m/lib/Drupal/system/Tests/Condition/ConditionFormTest.php

...stem/lib/Drupal/system/Tests/Plugin/ContextPluginTest.php

...s/system/tests/modules/condition_test/condition_test.info