Fixed .desktop file conformity

We currently

```bash
# Fix desktopfile
sed -i "s/Icon\=springlobby.svg/Icon\=springlobby/" %{buildroot}%{_datadir}/applications/%{name}.desktop
```

at @openSUSE games so it might be time to finally push this upstream. I am not sure if the `X-SUSE...` stuff is really necessary.