If _NET_WM_FULLSCREEN_MONITORS is set then the property needs to be updated because it can't
be deleted by sending a _NET_WM_FULLSCREEN_MONITORS client message to the root window
according to the wm-spec version 1.5. Having the window spanning more than two monitors also
needs the property set. In other cases, _NET_WM_FULLSCREEN_MONITORS doesn't need to be set.
What's more, setting _NET_WM_FULLSCREEN_MONITORS adds a constraint on Mutter so that such a
window can't be moved to another monitor by using the Shift+Super+Up/Down/Left/Right
shortcut. So the property should be added only when necessary.