QtWayland.Compositor: output fractional scaling

Qt based KDE landed fractional scaling support. I think this is something that should be in Qt.WaylandCompositor as well, currently a WaylandOutput{} has scaleFactor of type int: and can be set to any non-float value, depending on the screen a scaleFactor of 1 might be too small and a scaleFactor of 2 would be to big. Being able to set it to 1.7 may be optimal.