[kcmkwin/kwindecoration] Fix Get Hot New Stuff

Summary:
Found better categories for GHNS v3: looks like only Aurorae is supported?

Also refresh the themes when you close the GHNS window, in case something was downloaded.

Test Plan: Open GHNS, download new theme, close GHNS -> new theme visible

Reviewers: leinir, graesslin, KWin, davidedmundson

Reviewed By: leinir, KWin, davidedmundson

Subscribers: ngraham, cfeck, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18935