Change default sort order in the download dialog to Rating

Summary:
This changed the default sort ordering of the download dialog to Rating, for the following reasons:

• It's more relevant and useful to users to see content that the community thinks is best by default instead of simply what's been uploaded most recently
• Match Discover's default sort ordering for KNS content

Test Plan:
Open a GHNS dialog and see that the default sort ordering of the content is by Rating, and that the Rating radio button is checked:

Reviewers: apol, leinir, KNewStuff

Reviewed By: leinir, KNewStuff

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14097