The icon is meant as an addition to or replacement of the label. Icon
bitmaps for various states of the control (off, on, partially on, each
enabled or disabled, plus up to 125 custom states) can be set
individually via SetIconBitmap() (getter IconBitmap()).
The convenience method SetIcon() can be used to set the bitmaps for the
standard states from a single bitmap; it also supports cropping the
icon to its non-transparent area. Code borrowed from BIconButton.
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