[windowswitcher] Implement keyboard navigation
Summary:
Implements keyboard navigation via the up and down arrow keys for the Breeze window switcher.
Support for this went in with D16693.
CCBUG: 370185
Test Plan: Alt-tab, use arrow keys, it works!
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D16886