Issue #1966424 by tim.plunkett, dawehner, damiankloip: Allow Views handlers to be optional.

...dules/views/lib/Drupal/views/Plugin/views/HandlerBase.php

...b/Drupal/views/Plugin/views/display/DisplayPluginBase.php

.../Drupal/views/Plugin/views/exposed_form/InputRequired.php

...views/lib/Drupal/views/Plugin/views/row/RowPluginBase.php

...ews/lib/Drupal/views/Plugin/views/sort/GroupByNumeric.php

...es/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php

...s/views/lib/Drupal/views/Tests/Handler/HandlerAllTest.php

...ules/views/lib/Drupal/views/Tests/Handler/HandlerTest.php

core/modules/views/lib/Drupal/views/Tests/ModuleTest.php

core/modules/views/views.module

...ews/views_ui/lib/Drupal/views_ui/Form/Ajax/ConfigItem.php

...iews_ui/lib/Drupal/views_ui/Form/Ajax/ConfigItemGroup.php

core/modules/views/views_ui/lib/Drupal/views_ui/ViewUI.php