Port DesktopGrid QtQuickWindow to EffectQuickView

Summary:
Lots of red, does the same thing in a simplified way avoiding creating a
real window and having to hack round it.

Test Plan: Click add and remove, remove button disables appropriately

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: meven, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26381