Code review comment for lp:~seb128/pkgbinarymangler/keywords

I don't know why this has conflicts all over the place, but http://bazaar.launchpad.net/~seb128/pkgbinarymangler/keywords/revision/243 looks ok, except I would strip both the old and the new keyword header from .desktop files and extend the test to have both headers -- mainly in case there are still .desktop files with the old header in the archive or out of archive (unlikely to use pkgbinarymangler though), or for backports (unlikely too).