[Kickoff] Always select previous category in Applications View when navigating backwards

Summary: Current version of the code positions view on the previous category but doesn't set the category itself. I think that selecting the previous category and not the first one is more convenient for keyboard navigation (when you accidentally miss the right category, for example).

Reviewers: #plasma, ngraham, hein

Reviewed By: #plasma, ngraham, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22866