JCheckBox is often used as a cell renderer in tables and lists.
The specifications for the Java Look and Feel (codenamed Metal)
and the Windows Look and Feel specify that the
check box be rendered with a 2D flat border rather than a 3D
border when used as a cell renderer. The borderPaintedFlat property has
been added to enable this behavior.
This was an oversite of the original implementation. Unfortunately,
since ButtonModel is an interface, this property cannot be added to
it. It has instead been added to DefaultButtonModel, which should
help most developers who want this.