Issue #2328061 by tim.plunkett: Move datetime's FormElement #type classes in Core.

core/config/schema/core.data_types.schema.yml

core/includes/theme.inc

core/lib/Drupal/Core/Datetime/DateFormatInterface.php

core/lib/Drupal/Core/Datetime/DateHelper.php

core/lib/Drupal/Core/Datetime/Element/DateElementBase.php

core/lib/Drupal/Core/Datetime/Element/Datelist.php

core/lib/Drupal/Core/Datetime/Element/Datetime.php

core/lib/Drupal/Core/Datetime/Entity/DateFormat.php

core/lib/Drupal/Core/Datetime/Plugin/README.txt

.../config_translation/src/Tests/ConfigTranslationUiTest.php

core/modules/datetime/datetime.module

.../modules/locale/src/Tests/LocaleConfigTranslationTest.php

...grate/src/Plugin/migrate/destination/EntityDateFormat.php

...les/migrate_drupal/src/Tests/d6/MigrateDateFormatTest.php

...dules/system/config/install/core.date_format.fallback.yml

...ules/system/config/install/core.date_format.html_date.yml

.../system/config/install/core.date_format.html_datetime.yml

...les/system/config/install/core.date_format.html_month.yml

...ules/system/config/install/core.date_format.html_time.yml

...ules/system/config/install/core.date_format.html_week.yml