[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