Allow launching a tour while limiting it to once per-user. The idea is that GuidedTour handles this, so the calling code (GettingStarted, VisualEditor, etc.) doesn't need to think about hidden user preferences, etc. I'll call it launchOnceForUser as illustration; better name suggestions welcome
Example use case: The VisualEditor edit screen calls launchOncePerUser, then GuidedTour decides internally whether to launch it based on whether the user has already seen it.
Ideally, it will allow launching immediately, and pre-launching by setting a cookie (we did this once with GettingStarted).