Sets a window's label. The window's natural minimum size might be different after the label is changed, but the window's mininum size is not recomputed.
See get-label for more information.
Sets the bitmap label for a bitmap button. Since label is a bitmap, the bitmap must be valid (see ok? in bitmap%) and not installed in a bitmap-dc% object; otherwise, an exn:application:mismatch exception is raised. The bitmap label is installed only if the control was originally created with a bitmap label.
If l is #f, the window's label is removed.