Use new xpm icon

This addresses Debian bug #432791(please don't simply use Debian logo as icon) by combining the Debian logo already in the package with the ladybug image that also is in the package.

The resulting file is saved as rng.xpm, thereby also fixing the Lintian error http://lintian.debian.org/tags/menu-icon-not-in-xpm-format.html by using xpm format. The now unused debian-logo.png is deleted from the source.