Cleanup virutal desktops in deleted
Summary:
virtual desktops can be destroyed, active clients update, but deleted
keeps a cache.
Someone needs to do cleanup to avoid dangly pointers.
Test Plan:
I couldn't find a case of someone calling desktops mid way through an
animation, so it's only a hypothetical bug.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: graesslin, zzag, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D17247