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: ppy/osu#26673 (reply in thread)). 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.