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.
Note that I found this component to be fairly broken on KF6 in mobile mode: #52