Issue #2228093 by dawehner | sun: Modernize theme initialization.

core/config/install/core.extension.yml

core/core.services.yml

core/includes/common.inc

core/includes/errors.inc

core/includes/form.inc

core/includes/theme.inc

core/includes/theme.maintenance.inc

core/lib/Drupal.php

...Core/EventSubscriber/ThemeNegotiatorRequestSubscriber.php

core/lib/Drupal/Core/Extension/ModuleHandler.php

core/lib/Drupal/Core/Extension/ThemeHandler.php

core/lib/Drupal/Core/Form/FormBuilder.php

core/lib/Drupal/Core/Theme/ActiveTheme.php

core/lib/Drupal/Core/Theme/DefaultNegotiator.php

core/lib/Drupal/Core/Theme/Registry.php

core/lib/Drupal/Core/Theme/ThemeInitialization.php

core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php

core/lib/Drupal/Core/Theme/ThemeManager.php

core/lib/Drupal/Core/Theme/ThemeManagerInterface.php

core/modules/block/block.module