Make the attached property a simple QObject

Summary:
move all the logic in an attached property QObject
it's a bit less heavy to create so should make things slightly better
when this is used in delegates

Test Plan: Plasmoid background removal (with colors inversion) still works

Reviewers: bshah, Plasma

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27200