Add a hook to alter entity form displays

Problem/Motivation

https://www.drupal.org/project/field_readonly is a module which alters each entity form, to disable some of the form fields.
It used to have hook_field_attach_form for that.

Sadly there is no such hook in Drupal8, though there is \Drupal::moduleHandler()->alter('entity_display_build', $build_list[$key], $context);

Proposed resolution

Add \Drupal::moduleHandler()->alter('entity_form_build', $form, $context); into \Drupal\Core\Entity\Entity\EntityFormDisplay::buildForm

Remaining tasks

User interface changes

API changes

Data model changes