make color gradient of custom title bar adjustable (bug #44435) * settings-dialog.ui: new spinbox for 3D effect of custom title bars * settings-dialog.cc (settings_dialog): get settings for 3D effect and initialize the spin box; (write_changed_settings): write current spin box value into settings file; * octave-dock-widget.h: new class variable for 3D effect * octave-dock-widget.cc (octave_dock_widget): init new class variable; (set_style): set the gradient depending on class variable; (handle_settings): get settings on 3D effect from settings files