Comment 5 for bug 1490108

This was due to the fact there were two entries for different architectures for Steam. One for x86_64 and another for i686. Untoggling the check box caused both to appear as they both had the proprietary class.

I've committed a fix to address this.
https://bitbucket.org/lah7/ubuntu-mate-welcome/commits/08819c6444b62d5642c5a64f1bebef9ab3365696