improve exit sequence for GUI * octave-link.h, octave-link.cc (octave_link::accepting_events): Delete variable and all uses. (octave_link::link_enabled): New data member. (octave_link::octave_link): Don't set octave_exit. Initialize link_enabled. (octave_link::do_exit): Delete definition. Now pure virtual. Return bool. (octave_link::exit): Call instance->do_exit. (octave_link::enabled): New function. (ocave_link::process_events): New arg, disable. Optionally disable event processing. Use octave_link::enabled instead of instance_ok everywhere except for octave_link::exit. (octave_link::cleanup_instance): Delete. * octave-qt-link.h, octave-qt-link.cc (octave_qt_link::octave_qt_link): Accept thread as argument. Don't connect main_thread::finished signal. (octave_qt_link::~octave_qt_link): Don't delete main_thread. (octave_qt_link::do_exit): Emit exit_signal and return true. (octave_qt_link::exit_signal: New signal. (octave_qt_link::void octave_thread_finished_signal): Delete. * main-window.h, main-window.cc (main_window::_octave_main_thread): New member variable. (main_window::main_window): Initialize _octave_main_thread and _octave_qt_link to 0. (main_window::~main_window): Don't call octave_link::connect_link. Delete _octave_main_thread. (main_window::exit): Accept exit status as argument and call QApplication::exit instead of quit. (main_window::construct): Don't connect qApp::aboutToQuit to main_window::prepare_to_exit. (main_window::construct_octave_qt_link): Create _octave_main_thread and pass to _octave_qt_link. Don't connect _octave_qt_link::octave_thread_finished to main_window::exit. Connect _octave_qt_link::exit_signal to main_window::exit. * toplev.h, toplev.cc (main_loop): If quitting_gracefully, just return exit status instead of calling clean_up_and_exit. (do_octave_atexit): Now static. Call octave_link::process_events with disable arg set to true. (octave_atexit_functions): Now static. (clean_up_and_exit): New argument, safe_to_return. Call octave_link::exit and possibly return or wait for octave_link::exit to terminate the process. * octave.cc (octave_execute_interpreter): Don't alter return value from main_loop. Pass safe_to_return = true to clean_up_and_exit. Return retval instead of 0. (octave_initialize_interpreter): Don't call atexit.