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