[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