update xfburn.desktop.in file

Hi, I from Debian community. I used the lintian tool and encountered a message indicating that xfburn: desktop-entry-lacks-keywords-entry for [usr/share/applications/xfburn.desktop]. This commit fix it. Thank you! This field optional, but maybe can be useful.

Also fix xfburn: desktop-mime-but-no-exec-code for xfburn.desktop (add %F)

Also fix typos in .po files