Name processing threads

- PiiThreadedProcessor and PiiMultiThreadedProcessor now give spawned
  threads names according to the parent operation. This eases performance
  profiling using operating system tools such as "top".
Ignore state change events from disabled child operations

- When restarting a paused engine, child operations may end up being
  temporarily disabled. They change state to stopped, which made the parent
  erroneously change state to stopping.