`false` does not necessarily mean that validation failed. It just means that the value wasn't set. Should we differentiate? I think we can still change the `PreferencesSystem.set` API at this point because nothing in Brackets relies on that return value, and I'd be surprised if extensions used that yet. In fact, your use here is the first use I've seen of the `set` return value.

With the API as it stands now, a `false` return value means that the caller will have to call a separate function to find out if the given value is valid.