Each icon FOO comes in four variants:

FOO.xpm:    color, sensitive (original source)
FOO-xx.xpm: color, insensitive
FOO.xbm:    monochrome, sensitive
FOO-xx.xbm: monochrome, insensitive

If only captions or images are to be shown, DDD uses the
`buttonCaptionGeometry' and `buttonImageGeometry' resources to create
sub-images.  The default values are:

Ddd*buttonCaptionGeometry: 29x7+0-0
Ddd*buttonImageGeometry:   25x21+2+0

