Issue #1875992 by amateescu, swentel, yched: Add EntityFormDisplay objects for entity forms.

core/includes/entity.api.php

core/includes/entity.inc

core/lib/Drupal/Core/Entity/EntityFormController.php

.../lib/Drupal/Core/Entity/EntityFormControllerInterface.php

core/lib/Drupal/Core/Entity/EntityFormControllerNG.php

core/modules/block/block.install

core/modules/block/custom_block/custom_block.module

...ck/lib/Drupal/custom_block/Tests/CustomBlockFieldTest.php

core/modules/comment/comment.module

core/modules/datetime/datetime.module

...l/datetime/Plugin/field/widget/DatetimeDatelistWidget.php

...al/datetime/Plugin/field/widget/DatetimeDefaultWidget.php

.../datetime/lib/Drupal/datetime/Tests/DatetimeFieldTest.php

core/modules/edit/lib/Drupal/edit/Form/EditFieldForm.php

core/modules/edit/lib/Drupal/edit/Tests/EditTestBase.php

core/modules/email/lib/Drupal/email/Tests/EmailFieldTest.php

core/modules/email/lib/Drupal/email/Tests/EmailItemTest.php

core/modules/entity/entity.install

core/modules/entity/entity.module

core/modules/entity/lib/Drupal/entity/EntityDisplayBase.php