Enable adding listeners to event before the event is registered #97
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Currently, if listeners are added before the event they are meant to watch is added, the listener gets removed when the event is registered. We should keep them around.
This showed up in work around jupyter server.
If one extension (E1) wants to listen to events from another extension (E2), a problem arises is E2 is loaded after E1 is loaded. This is a common use-case. jupyter server doesn't have a dependency injection system yet, so we can't name extension dependencies.
Either way, I think the event logger show allow listeners to be added even if the event isn't registered yet. It's a no-op until the event is registered.