[Xmonad] focus fixes for Circle and MagicFocus

These patches fix Circle and MagicFocus, they both assume the focus window in a Stack actually has the focus, which is not true with floating windows or multiple screens. But I don't use floating windows or multiple screens, so I never noticed... (Actually, I don't use MagicFocus anymore, and I haven't heard anybody saying they do. Maybe it should be removed?)