Add a preview for the image split dialog.

It can be very confusing to figure out what horizontal and vertical guides
mean, especially if you're doing a lot of programming. As this feature is
most commonly used to prepare graphics for games and other interactive media,
programming might be commonly done by those using this feature.

(cherry picked from commit 87f7521e51dc40b4603a873a36081c6b84125f2c)