Summary:
Removes over 100 lines of rather confusing code, implicitly introduces a
sharedEngine. Doesn't perform hide/show by moving a window offscreen.
As for the freeze, proving that EffectQuickView covers this case without
lots of extra C++ is an important pre-requisite for ever moving this
effect to a magic declarative API.
General code structure remains the same.
Test Plan: Opened present windows
Reviewers: #kwin, zzag
Reviewed By: #kwin, zzag
Subscribers: zzag, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D24240