Add KNS ("Get New Stuff") support to deKorator

Frank (of openDesktop.org fame) was kind enough to add the GNS provider entries to kde-look.org for deKorator themes, and I quickly tested it with some themes.

Unfortunately, many themes are not following the guidelines for archive names or contents, so after installing them, they do not show up in the theme list.

I will improve "broken" theme file detection in the future.