Add mutex lock to shutdown confirmation for proper thread timing (bug #44751). * main-window.cc (main_window::confirm_shutdown_octave): Lock Qt link mutex before confirming shutdown thereby ensuring link is in sleep state. Unlock mutex after confirming shutdown, then awake all threads waiting on the mutex. * dialog.cc (QUIWidgetCreator::dialog_button_clicked): Block threads by locking mutex before setting dialog_button and dialog_result. Unlock mutex afterward. (QUIWidgetCreator::list_select_finished): Lock mutex before setting list_index and dialog_result. Unlock mutex afterward. (QUIWidgetCreator::input_finished): Lock mutex before setting string_list and dialog_result. Unlock mutex afterward. (QUIWidgetCreator::filedialog_finished): Lock mutex before setting path_name, string_list and dialog_result. Unlock mutex afterward. * libgui/src/dialog.h (QUIWidgetCreator::wait): Remove function and make the QMutex (mutex) and QWaitCondition (waitcondition) available to public. * octave-qt-link.cc (octave_qt_link::octave_qt_link): Move the octave_qt_link object affinity to the main_thread. (octave_qt_link::do_confirm_shutdown): Lock mutex before emitting signal so that GUI thread waits until mutex is free. Unlock the mutex after coming out of sleep state. (octave_qt_link::do_prompt_new_edit_file): Lock the uiwidget_creator mutex before emitting signal and go to sleep after emitting signal. Unlock mutex after mutually shared data has been read. (octave_qt_link::do_message_dialog): Ditto. (octave_qt_link::do_question_dialog): Ditto. (octave_qt_link::do_list_dialog): Ditto. (octave_qt_link::do_input_dialog): Ditto. (octave_qt_link::do_file_dialog): Ditto. (octave_qt_link::do_debug_cd_or_addpath_error): Ditto.