[effects/fadedesktop] Properly check whether given window is on a particular virtual desktop
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.
Switched between virtual desktops with a Konsole window being on all
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Differential Revision: https://phabricator.kde.org/D16103