Minor improvement of the desktop file

It is the same as #553 but without i18n.
Thunar, the file manager of Xfce, says me the launcher is not trustable and does not show the icon if it is not executable. On the screenshot (in French), I have 3 choices : launch anyway, mark as executable, cancel. It may be specific to Thunar (at least pcmanfm does not need desktop files to be executables).
![capture d cran - 06072014 - 23 12 08](https://cloud.githubusercontent.com/assets/4244587/3489988/c70e5e70-0552-11e4-9adf-577b5f7f4a48.png)