Ordered: provide insert_at(), remove_from(), and update_from_to() to allow redefinition of add(), remove() and update() by subclass
Control.Radio:
  - implement add(), remove(), and update(), using Ordered..insert_at(), remove_from() and update_from_to()
  - use emit_add(), emit_remove(), and emit_update() to update destination based on single value change
  - implement fetch() as this is a stateful pipelet, retreiving the current value, if any (not tested)
  - ToDo: probably move this code to SingleChoice
  - ToDo: implement update_() to avoid loosing selected value when this is only a value change, but not a position change