Field Widgets Need $langcode variable

This issue is a side-effect of #367595: Translatable fields. When we added translatable fields, we started passing $langcode into all the field-level hooks. However I discovered that we're not passing this into any of the widget-level hooks. In the case of hook_widget(), this poses a serious problem.

In file.module, we need access to the current value of the field from $form_state['values']. Previously we were using:

  if (isset($form_state['values'][$field_name])) {
    $items = $form_state['values'][$field_name];
  }

However $form_state['values'][$field_name] no longer contains the needed items, it's in $form_state['values'][$field_name][$langcode]. However, $langcode is not known and should be passed into this hook.

This functionality will be needed in any field that contains AJAX behaviors, when the field value needs to be determined entirely by the values for $form_state['values'].