PythonIPCPseudoConduit::registerCallbackForCode doesn't work

Assigned to Tender discussion #60.

I should have been clearer in my original description: registerCallbackForCode doesn't have the intended effect specifically when the other end is an EventStreamConduit, which is the case for both the client scripting bridge and the serverside conduit plugin. If the remote end is another type of conduit (say, a SimpleConduit) that sends events regardless of whether the local end has asked for them, then registerCallbackForCode will work fine.

The fix is probably to add a variant of SystemEventFactory::setEventForwardingControl that takes a code rather than a name string, and then update EventStreamConduit to handle forward-by-code.

While I still think that this is the best way to resolve this issue, a good intermediate fix would be for CodecAwareConduit to convert code-based registrations into name-based ones. That would work for all events except those without tag names (i.e. system events), but we could at least throw an appropriate error in that case.