OTS11 — Actions and removal

In the Additional Structuring Mechanisms for the OTS, an Action can register
interest in more than one SignalSet, i.e., it can be passed to add_action
more than once, with a different SignalSet parameter. However, remove_action
is not parameterised on the SignalSet, so currently it must remove the
Action from all registered SignalSets. We can either add the SignalSet as a
parameter to remove_action or add another method, leaving the semantics of
remove_action as they are

close no change