SwipeListItem: Improve RTL (mirrored) layout

 - Use listItem.mirrored consistently, port all the other ways of
   detection to it. Not only it avoids creating extra unnecessary
   attached objects, but also prevents layouting issues as explained
   next.
 - Do not trust implicit inheritance, because even if
   LayoutMirroring.childrenInherit is set to false, this internal
   content should look and behave as mirrored, which is the whole point
   of setting LayoutMirroring.enable: true on the list item itself.