[suggestion] QtWayland.Compositor: change multi-screen behaviour

Instead of each surface being on each output, it would be nice if the whole compositor is a "space".
Basically what I'm proposing is the approach of

            Repeater
            {
                model: shellSurfaces
                ShellSurfaceItem
                {
                    autoCreatePopupItems: true
                    shellSurface: modelData
                }
          }

to be parent-able not only to a window of an output but to the compositor itself and the shellsurfaceitem to be draggable accross WaylandOutputs.