Restore active client after ending showing desktop

Summary:
Showing desktop requests focus on the desktop window. This means the
active window is reset. When ending showing desktop the state was not
restored.

This change addresses this problem by requesting focus to the best
window.

BUG: 375993
FIXED-IN: 5.10.4

Test Plan: New autotest and manual testing

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6420