Translate items in descending z order.

This adds z translation to items so that earlier list items are stacked on top of later list items. My specific usage case is for list items that contain dropdown menus. 

Without this change, such menus are hidden underneath later list elements, and to solve this would require (to my knowledge) either layered (and so non-scrolling) dropdowns, or event-handling menus from outside the `<core-list>`, which is a more complicated solution than this.