Bug#591812: lintian test for menu icon type is out of date

[bdale@gag.com (Bdale Garbee)]

Package: lintian

While the packaging manual section 3.7 suggests that Debian menu requires an
xpm format icon, that is not actually true.  In all cases I've tried, a png
is just fine too.  Several packages now deliver png icons that work fine with
both desktop and menu files.

Currently, lintian emits the error:

	E: openrocket: menu-icon-not-in-xpm-format

when it finds a non-xpm icon file.  This seems inappropriate given that png
files actually seem to work fine.

Please consider downgrading this to a warning, or updating the set of file
types to include png.

Obviously, the packaging manual should also be updated, but I'm not sure 
offhand where responsibility for that lies.

Bdale