[effects/fadedesktop] Properly check whether given window is on a particular virtual desktop

Summary:
In order to check whether a window is on some particular virtual
desktop, one has to use isOnDesktop.

Also, this change adds implicit support for the new virtual desktop protocol.

Test Plan:
Switched between virtual desktops with a Konsole window being on all
desktops.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16103