Simplify Toplevel::isOnDesktop() and Toplevel::isOnAllDesktops()
Wayland- and X11-specific code paths are not needed.
If the window is on all virtual desktops, Toplevel::desktops() will be an empty list.
If the window is on a particular virtual desktop, Toplevel::desktops() will contain that desktop. On X11, the desktops list can have at most only one item.
Edited by Vlad Zahorodnii