Adhere to OpenDesktop menu-spec

If the `.desktop` file is invalid according to http://standards.freedesktop.org/desktop-entry-spec/1.0/ it will break rpm on http://build.opensuse.org

```
ERROR: No sufficient Category definition: /home/abuild/rpmbuild/BUILDROOT/Desurium-0.8.0_rc4-0.i386//usr/share/applications/desura.desktop 
WARNING: file extension in Icon line: /home/abuild/rpmbuild/BUILDROOT/Desurium-0.8.0_rc4-0.i386//usr/share/applications/desura.desktop
ERROR: Icon file not installed: /home/abuild/rpmbuild/BUILDROOT/Desurium-0.8.0_rc4-0.i386//usr/share/applications/desura.desktop (../../share/desurium/desura.png)
WARNING: Empty GenericName: /home/abuild/rpmbuild/BUILDROOT/Desurium-0.8.0_rc4-0.i386//usr/share/applications/desura.desktop
Errors in installed desktop file detected. Please refer to http://en.opensuse.org/SUSE_Package_Conventions/RPM_Macros
error: Bad exit status from /var/tmp/rpm-tmp.4K6kkj (%install)
```

This should fix it.