Update link type settings in administration using ajax subform

Currently, changing the link type settings from "Normal link" or "AJAX link" to "Confirm Form" creates a validation error when saving because the settings fields "flag_confirmation" and "unflag_confirmation" are available only when "Confirm form" is already chosen. Additionally, changing the link type settings from "Confirm Form" to "Normal link" or "AJAX link" will trigger the HTML5 "required" attribute, because the unrelated settings fields are not replaced. (admin/structure/flags/manage/label > "Display settings" > "Link type")

This patch provides that the settings fields are replaced on change using the Drupal FormAPI AJAX settings.
With this, changing and saving the link type settings will succeed.