Reverted removal of EntityEmbedCKEditorEmbedForm::validateForm().
Added documentation for EntityHelperTrait.
Fix errors not displayed if entity type not selected. Check $entity->access() when the user embeds the entity.
Moved the file access workaround to the EntityHelperTrait.
Merge branch 'entity-manager-helper' into dialog-multistep-form

Conflicts:
	src/Form/EntityEmbedCKEditorSelectForm.php