Transitioner: Short-circuit SelectionChanged event when not originating from Transitioner

[nicolaihenriksen]

Potentially fixes #3268

Effectively short-circuits the SelectionChanged event from bubbling up from the Transitioner when the original source of the event is not the Transitioner itself. Please consider if you see any negative side-effects to this; I could not really see any, but I would like your input on that as well.