ContextMenu MenuItem templates missing scrollviewers

[CBeatt13]

MenuItem styles are missing ScrollViewers used by ContextMenu when the desired height of the ContextMenus items are less than the menu's height. By default when this happens Context Menu's provide an up/down arrow to allow the user to scroll through the menu to access items not yet visible. The styles provided do not have this functionality and if too many items are in the context menu the rest simply don't render leaving no way for the user to access the additional items.