Tablet input should have a property to allow adjusting the mapping target area

[peppy]

A common user request has been to add back tablet support for letterbox modes (example: https://github.com/ppy/osu/discussions/26673#discussioncomment-8223467). Basically, this sees the tablet area mapped to the scaled screen size (when "Everything" scaling is used in osu!).

To make this work, the TabletHandler should have a bindable property that defines the target area, which osu! can adjust based on its scaling mode.