[effects/slidingpopups] Port away from deprecated EffectWindow::desktop

Summary:
If a window is not on the current virtual desktop, then it won't be
painted/slided. Thus, we can replace EffectWindow::desktop by
EffectsHandler::currentDesktop.

Test Plan: The effect still works correctly on both X11 and Wayland.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17944