Move file_entity and file_browser's Field Widget for Files and Images to Entity Browser

There are many options I can think of, none of which are perfect. My best idea is probably to:

1) Create a new Field Widget in Entity Browser that contains the best of both existing widgets.
2) Remove the body of both widgets in File Browser and Entity Browser and have them extend the new Entity Browser Field Widget.
3) Use a form alter in both File browser and Entity Browser to hide the deprecated widgets from the Form Display tab (unless it was already selected).

This has the benefit of not confusing new users with three widgets, and not requiring any action for existing users.

Thoughts?

So I've been working no #3.2 and #3.3 by implementing a funny isApplicable method which only returns TRUE if the field definition is already using the file_browser field widget. It's pretty unique, so I'd like to get feedback on it as a solution for both File Browser and File Entity:

  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    // If the bundle this field belongs to uses File Browser in any existing
    // form display, we're still applicable. This lets users who are already
    // using this widget to continue their work normally, but prevents future
    // users from using this instead of Entity Browser's widget.
    $entity_type_id = $field_definition->getTargetEntityTypeId();
    $bundle = $field_definition->getTargetBundle();
    $ids = \Drupal::entityQuery('entity_form_display')
      ->condition('bundle', $bundle)
      ->condition('targetEntityType', $entity_type_id)
      ->execute();
    $displays = EntityFormDisplay::loadMultiple($ids);
    /** @var EntityFormDisplay $display */
    foreach ($displays as $display) {
      $configuration = $display->getComponent($field_definition->getName());
      if (isset($configuration['type']) && $configuration['type'] == 'file_browser') {
        return TRUE;
      }
    }
    return FALSE;
  }

The alternative to this from my understanding is a form alter which checks if file_browser/file_entity_browser is already selected, and if it's not hide it from the dropdown.

Here's a first shot at this. My notes:

1. Updated the example module's content type to include a new Image field which uses this widget and has some picky field settings.
2. Made the concept of passing validators and selected entities to displayEntityBrowser()generic so that developers can pass any data they want to persist between reloads.
3. Added the "File" WidgetValidation plugin, which calls the normal file_validate() function. Core constraints were not used here as the formatting of our values can not easily match what core's FileValidationConstraintValidator expects. We typically deal with arrays of entities, and core expects a FileItem, which is hard to generate in non-field use-cases like form elements and CKEditor.
4. Added the "File" FieldWidget, which is mostly copy+pasted from a hybrid of File Entity and File Browser's codebases.
5. In WidgetBase, I added a getForm() implementation that child classes can call to override Widget configuration on-the-fly using a "widget_context" provided by something high up the chain (in this case, the field widget). This is useful for other use cases because now there's a generic way to override Widget configuration based on where it's used, without hard-coding anything. If widgets don't call WidgetBase::getForm(), they can still use widget_context or any other form state key in creative ways.
6. As a result of the previous note, the File setting "upload_location" is now respected by the Upload widget.

I'm putting the issue into needs review so I can get some eyes on it.

Fix tests.

@Berdir Agreed and fixed. Changing the EntityReference class name would cause many other problems (like breaking every contrib Field Widget) so I think we should address that in a new issue.

#11.1/#11.2 - Done.

#11.3 - I did both - hid the settings form element if the field type wasn't image, made the description for "preview_image_style" more descriptive and hid the image style from the settings summary if it isn't relevant.

Interdiff is huge because I realized that I change the class name of the Field Widget without changing the file name.

Stupid mistake, for some reason I remembered #hidden being a valid form api setting.

#2767373: FileBrowserWidget improperly limits validation errors on the remove button. is a quick follow up bugfix that could use a review/commit.