Minor improvement of the desktop file

It is the same as #553 but without i18n.
$ ls -l
total 16
-rwxr-xr-x 1 rydroid rydroid   169 juil.  6 22:57 Dolphin Emulator.desktop
-rwxr-xr-x 1 rydroid rydroid 10922 juil.  6 22:57 gimp.desktop
$ ./Dolphin\ Emulator.desktop 
./Dolphin Emulator.desktop: ligne 1: [Desktop : commande introuvable
./Dolphin Emulator.desktop: ligne 5: Wii/GameCube: Aucun fichier ou dossier de ce type
$ ./gimp.desktop
$
$ # In this example, "Dolphin Emulator.desktop" without xdg-open does not launch, but "gimp.desktop" that has the xdg-open launchs. I agree that the usefulness is not big at all, but there may be some people that are used to launch apps with desktop files in the terminal, who knows? In my case (Debian GNU/Linux 8 with Xfce), all my desktop files have this line.