Revert "Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system."

This reverts commit fc04601c.

Issue #2004086 by dawehner, pwolanin, katbailey, tim.plunkett, vijaycs85, fabpot: The Request service must be synthetic.

Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Issue #1886448 by dawehner, sun, Berdir, ParisLiakos: Rewrite the theme registry into a proper service.

Issue #2073531 by tim.plunkett, dawehner, Berdir, catch, plach: Use current user service instead of _account, remove _account from the request object.

Issue #1798296 by damiankloip, Xano, David_Rothstein, Crell, dawehner: Integrate CSRF link token directly into routing system.

Issue #2084463 by dawehner, pwolanin, David_Rothstein, tim.plunkett, Xano: Convert contextual links to a plugin system similar to local tasks/actions.

Issue #2019055 by plach, fago, berdir, yched, Gábor Hojtsy, kfritsche, jibran: Switch from field-level language fallback to entity-level language fallback.

Revert "Issue #2019055 by plach, fago, kfritsche: Switch from field-level language fallback to entity-level language fallback."

This reverts commit 46942cd3.

Issue #2019055 by plach, fago, kfritsche: Switch from field-level language fallback to entity-level language fallback.

Issue #2048223 by dawehner, ParisLiakos, herom, fubhy, damiankloip, vijaycs85, joelpittet, tim.plunkett: Add $account argument to AccessCheckInterface::access() method and use the current_user() service.

Issue #2057259 by alexpott, cweagans, swentel: Fixed Malformed modulename.info.yml file causes fatal error.

Issue #2102125 by dawehner, tim.plunkett, neclimdul, pwolanin, YesCT, damiankloip, Tim Bozeman: Big Local Task Conversion.

Issue #2107137 by dawehner, damiankloip, tim.plunkett: Fix the DX for declaring custom access checkers.

Issue #2050835 by Berdir | yched: Move Widget, Formatter, and FieldType plugin types to the Core\Field system.

Issue #2048827 by claudiu.cristea, tim.plunkett: Move Image toolkit API from system.module to Drupal\Core.

Issue #2077473 by dawehner, damiankloip, pwolanin: Replace local action plugin discovery with YamlDiscovery and handle routes with parameters.

Issue #2091411 by damiankloip, dawehner: Provide an easier mechanism for a route definition wrapped by module_exists().

This reverts commit 227f21b3.

Issue #1798738 by alexpott, mtift, tayzlor, ianmthomasuk, Albert Volkman, heyrocker, cam8001: Move css_js_query_string() to state system.

Issue #2070651 by pwolanin, dawehner, larowlan, tim.plunkett, ParisLiakos: Introduce a path-based breadcrumb builder, remove the one that's based on {menu_links()}.

Issue #1969572 by neclimdul, larowlan, hussainweb, dawehner, damiankloip, ParisLiakos, pwieck, Berdir, socketwench: Make Uuid a service.

Issue #2041885 by BTMash, cweagans, klausi: Move HTTP basic authentication provider to a separate module.

Issue #2076085 by dawehner, damiankloip, Letharion, pwolanin: Resolve the need for a 'title callback' using the route .

Issue #2076283 by dawehner, pwolanin, neclimdul: Allow local task plugins on paths with a dynamic value (like a node).

Issue #2089635 by tim.plunkett, disasm, larowlan: Convert non-test non-form page callbacks to routes and controllers.

Issue #1971158 by Berdir, beejeebus, alexpott, amateescu: Follow-up: Add loadMultiple() and listAll() caching to (cached) config storage.

Issue #2050919 by neclimdul, pwolanin, damiankloip, dawehner: Replace local task plugin discovery with YamlDiscovery.

Issue #2057869 by dawehner, jibran: Provide an alias for 'plugin.manager.entity' called 'entity.manager'.