Set up the icons for minimizing.
set_icon_list() allows you to pass in the same icon in several hand-drawn
sizes. The list should contain the natural sizes your icon is avilable in;
that is, don't scale the image before passing it. Scaling is postponed
until the last minute, when the desired final size is known, to allow best
quality.
By passing several sizes, you may improve the final image quality of the
icon, by reducing or eliminating automatic image scaling.
Recommended sizes to provide: 16x16, 32x32, 48x48 at minimum, and larger
images (64x64, 128x128) if you have them.