Improve keyboard navigation

Summary:
Since the tab view is dead, we should improve the keyboard navigation of the sidebar view.

BUG: 404065
FIXED-IN: 5.17.0

Test Plan: Played a bit with it, it looks like it's working.

Reviewers: Plasma, Plasma Accessibility, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23975