Draft: Avoid doing any extra work in Window::setDesktops() if the window is closed
Window::setDesktops() can be closed if a closed window needs to be evacuated from a removed virtual desktop. In that case, just update m_desktops and do nothing extra.
Regarding bug 486622, it used to be possible to activate a closed modal dialog. After xdg-dialog-v1 refactor changes, kwin should not hit that case anymore.