Backgrounds: Add choice to resize image to max fitting in the screen with no crop

[Starcrasher]

At the moment, Background allow to adjust the image. It gives good results most of time but, the method sometimes crops parts of the image without knowing if these parts were relevant.
Try that method with default image, Haiku logo, and you lose H & U.
So, I suggest to have an additional choice that would grow image size until it stops to fit entirely. That would leave some borders but with aid of manual placement users could even, in some cases, take advantage of it. For exanple, using the plain place to put icons or replicants.