Linux installer fails creating a desktop shortcut because it uses a hardcoded path to desktop

When running install.h in Arduino 1.6.6 Linux, an error occurs:

```
cp: cannot create regular file '/home/jonatan/Desktop/arduino.desktop': No such file or directory
```

It fails on localized systems where the Desktop's path is different. The correct path can be found in the variable XDG_DESKTOP_DIR in ~/.config/user-dirs.dirs.
