Config translation form elements output both the source and the translation build and submit values themselves (ElementInterface::getFormElement() no longer exists)

Form elements for configurationt translation were revamped to provide more control over the config translation form.

This has resulted in three changes to \Drupal\config_translation\FormElement\ElementInterface:

1. ElementInterface::create() has been introduced to formalize element construction. The new \Drupal\config_translation\FormElement\FormElementBase implements this method and generally does not need to be overridden.
2. ElementInterface::getFormElement() has been renamed to ElementInterface::getTranslationBuild(). The "translation" part is to allow for richer form elements in contrib which are not only related to translation. This method is now responsible for returning the entire sub-structure for the specific configuration element: both the source (which is provided to the translator for context) and the form element for entering the translation. A new \Drupal\config_translation\FormElement\FormElementBase is introduced which helps out with the related logic and provides base getSourceElement() and getTranslationElement() methods. The latter one needs to be overridden for each implementation.
3. A new ElementInterface::setConfig() is introduced to give form elements more control over setting the configuration values. The new \Drupal\config_translation\FormElement\FormElementBase implements this method and generally does not need to be overridden.

Take a look at \Drupal\config_translation\FormElement\Textfield for an example implementation of form elements which extend the base class.