Sets up the icon representing this window. This icon is used when the
window is minimized (also know as iconified). Some window managers or
desktop environments may also place it in the window frame, or display it
in other contexts.
The icon should be provided in whatever size it was naturally drawn; 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.
If you have your icon hand-drawn in multiple sizes, use set_icon_list().
Then the best size will be used.
GTK2.Window()->set_gravity()