RequiredFieldsApp: some cleanup

 * remove unused `type2thing`
 * rename `typeII` to `toggleSave`
 * bind `app.activated` directly to `toggleSave`
 * add `fieldValueIsBlank` helper function
 * add `anyValueIsBlank` helper function
 * add `App#checkedFieldValues` helper method
 * remove `App#getFieldValue` helper method