[client] Ensure PlasmaWindowManagement updates active window if it goes away

Summary:
So far when the active PlasmaWindow got unmapped or destroyed, the
PlasmaWindowManagement didn't update the activeWindow. This means it
could expose a deleted object through it's API which could result in
a crash.

This change addresses the problem by updating the active window when
a window gets unmapped or destroyed.

Test Plan: Tests added which exposed the problem

Reviewers: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D1621