dock-/undock- and close-icons with variable size (bug #41938) * widget-close-light.png, widget-close.png, widget-dock-light.png, widget-dock.png, widget-undock-light.png, widget-undock.png * widget-close-light.svg, widget-close.svg, widget-dock-light.svg, widget-dock.svg, widget-undock-light.svg, widget-undock.svg * octave-dock-widget.cc (octave_dock_widget): derive icon size from the small icon size that suits the actual qt style * octave-dock-widget.h: new class variable storing the icon size