[effects/cubeslide] Fix "sticky" windows detection on Wayland
Summary: This is second part of D15175: [effects/cubeslide] Fix visual glitches with Blur / BackgroundContrast effect, and aimed at fixing Bug 390366.
The problem was that effect relied on isManaged() to detect OSDs and notifications, that should be painted on top of the cube. On Wayland, the desktop window itself is not managed, which made it "sticky".
Instead we use isSpecialWindow() check to explicitly make notifications and OSDs sticky.
It still requires fixing Plasma::Dialog, since it does not set isSpecialWindow() flag correctly.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D15496
BUG: 390366
FIXED-IN: 5.15.0