Add an API to easily get access to the selection relative to the image (v2)

[DavidRouyer]

Is your feature request related to a problem? Please describe. Today, it's very difficult to get access to the selection relative to the image (x,y,width,height).

Describe the solution you'd like We could have something like $relativeSelection() which returns an object like {x, y, width, height} with values transformed from the x, y, width and height from the selection applied to the transform from the image.

Describe alternatives you've considered n/a

Additional context n/a

[fengyuanchen]

The selected x, y, width and height values are entirely based on the image.