Select an idle worker.

def select_idle_worker(self, ctrl, shift):
    """Select an idle worker."""
    action = sc_pb.Action()
    mod = sc_ui.ActionSelectIdleWorker
    if ctrl:
      select_worker = mod.AddAll if shift else mod.All
    else:
      select_worker = mod.Add if shift else mod.Set
    action.action_ui.select_idle_worker.type = select_worker
    return action