The kdiff3.desktop file, used for showing KDiff3 in XDG menus (like KDE's, GNOME's, XFCE's) is not totally compliant, as executing
desktop-file-validate on it shows. Furthermore, specifies no categories, so it could be not shown at all (or in some "lost+found" category).
The attached patch fixes the following problems:
- removes the deprecated %m marker from the Exec key
- renames the non-standard KDE-specific DocPath key to X-DocPath, as changed with KDE 4
- adds a list of two categories, "Qt" (as it for sure a Qt application) and "Development" (being a mostly-development tool)