Refactors SC.MenuPane slightly.

• Fixes a problem where the items array was not observed for content changes when it was set on create (includes unit test).
• Removes an unnecessary layout change in createChildViews, which also meant that creating a menu pane as a singleton (i.e. with no run loop) would throw warnings.
• Slightly simplifies the code by removing duplicated properties from the superclass SC.PickerPane and reusing the items observer code for initialization.
• WARNING: Removes the SC.MenuPane.VERTICAL_OFFSET property. SC.PickerPane already has a provision for offsetting panes from the window's edges, windowPadding, and that is the property that is being used now.