Manual cropping

[yephny]

Cropping is arguably the most essential feature of a scanner app, other than scanning documents. The app is doing the cropping on its own, which doesn't really come out good often. Being able to crop the images manually, though still with image recognition (so that the whole side of the rectangle would be moving to the line the paper makes with the background when I move a corner of the cropping rectangle) would be super useful.