Refactors SC.MenuScrollView.

• Reduces code by removing duplicate properties from its superclass SC.ScrollView.
• Renames verticalScrollerView & verticalScrollerView2 to topScrollerView & bottomScrollerView respectively to make it clearer which they are and to avoid confusion with the verticalScrollerView property of SC.ScrollView.
• Removes four (!) extra observers (one which was totally useless on the non-existant horizontal scroller visibility).
• Tidies up the createChildViews code to be more readable and not make excess calls to reposition the scrollers.
• Improves the appearance/disappearance of the scrollers. It now shifts the content view at the same time that the container view adjusts to show/hide scrollers so that the content view never appears to jump, which also ensures that there is no extra space at the bottom of the container view.