Fix a memory leak

Hello, we had some trouble with the garbage collector.
In some cases the listener on the ExclusiveTouch is not removed.
In the case of a scroll container (in INTERACTION_MODE_TOUCH the default value for interactionMode) with SCROLL_POLICY_OFF that contains a button that trigers a change of the current screen to a new one (disposing the scrollContainer), the scroll container would never go into the stage_touchHandler() / TouchPhase.ENDED event and would not remove the exclusive touch listener ExclusiveTouch.forStage(this.stage).removeEventListener(Event.CHANGE, exclusiveTouch_changeHandler);

I hope that helps,
you might ask why we use scrollcontainers with no scroll, it's just for the background option and its ease of using slice9images.