set up octave_link when running with --no-gui option (bug #44116) * main-window.h, main-window.cc (main_window::_start_gui): New member variable. (main_window::main_window): New argument, start_gui. Skip most initialization if start_gui is false. (main_window::confirm_shutdown_octave): Skip interactive confirmation if _start_gui is false. (main_window::connect_uiwidget_links, main_window::construct, main_window::construct_octave_qt_link): Skip most initialization if _start_gui is false. (main_window::handle_octave_ready): Handle non-gui case. * octave-gui.cc (octave_start_gui): Unify gui/non-gui options.