improve title bar of dock widgets * libgui/src/module.mk: new icon files for darker dock widget's title bars * libgui/src/resource.qrc: new icon files for darker dock widget's title bars * octave-dock-widget.cc (constructor): class vars for close action and icons; (set_title): transparent background of title text; (make_window, make_widget): new dock icon depends on chosen color; (handle_settings): title bar with gradient, chose icons depending on color * octave-dock-widget.h: new class variables