Restructure shutdown flow and behavior for improved robustness * file-editor-interface.h (file_editor_interface::check_closing): Remove closing_state argument from input. * file-editor-tab.cc (file_editor_tab::_cancelled): New static variable for managing chain of file closes. (file_editor_tab::file_editor_tab): Discard _app_closing and _modal_dialog initializations. (file_editor_tab::closeEvent): Upon accepting the close event, als request tab removal. (file_editor_tab::check_modified_file): New simple routine checking for modified file if there hasn't been cancellation yet. (file_editor_tab::run_file): Remove use of _modal_dialog. (file_editor_tab::show_dialog): Add modal input setting. Remove use of _app_closing and _modal_dialog. (file_editor_tab::check_file_modified): Remove situational message for just one. If modified, call show_dialog with modal true, return QMessageBox::Cancel if anything cancelled, otherwise the user decision. (file_editor_tab::handle_file_modified_answer): Do not remove file editor tab in any case. If user cancelled, set _cancelled true. (file_editor_tab::save_file): Add "false" (non-modal) to show_dialog() call. (file_editor_tab::save_file_as): Add "false" (non-modal) to show_dialog() call. (file_editor_tab::conditional_close): Remove input argument and use of variable _app_closing. * file-editor-tab.h (file_editor_tab::conditional_close): Remove input argument. (file_editor_tab::reset_cancel): New member function declaration for managing cancellation of tab chain saves. (file_editor_tab::was_cancel): New member function for indicating any cancellation when saving modified files. (file_editor_tab::check_modified_file): New slot declaration for checking and saving file modifications. (file_editor_tab::show_dialog): Add modal option input argument. (file_editor_tab::_app_closing, file_editor_tab::_modal_dialog): Remove. (file_editor_tab::_cancelled): Add. * file-editor.cc (file_editor::file_editor): Remove _check_closing_done initialization. (file_editor::~file_editor): Remove use of _check_closing_done and check_closing(). (file_editor::check_closing): Clear cancellation history and check all files for modifications, without closing. Close all tabs and delete associated widgets if no cancellation. Remove use of _check_closing_done. (file_editor::request_close_file): Remove argument from conditional_close() function call. (file_editor::request_close_all_files): Ditto. (file_editor::request_close_other_files): Ditto. (file_editor::handle_tab_close_request): Ditto. (file_editor::handle_tab_remove_request): Add comment about deleting sender. (file_editor::add_file_editor_tab): Connect fetab_check_modified_file signal to check_modified_file slot. * file-editor.h (file_editor::fetab_check_modified_file): New signal. (file_editor::check_closing): Remove input argument. (file_editor::_check_closing_done): Remove. * main-window.cc (main_window::confirm_shutdown_octave): Rename of member function main_window::confirm_exit_octave(). Change return type to void. Set octave/qt link confirmation result then awake worker process. (main_window::exit_app): Rename of member function main_window::exit(). (main_window::closeEvent): Rather than post callback event, queue "exit" in the command buffer. (main_window::construct_octave_qt_link): Connect confirm_shutdown_signal to confirm_shutdown_octave slot. Connect exit_app_signal to exit_app slot. (main_window::exit_callback): Remove. (main_window::confirm_exit_octave): Renamed as confirm_shutdown_octave(). * main-window.h (main_window::confirm_shutdown_octave): Rename of slot confirm_exit_octave(). (main_window::exit): Renamed exit_app(). (main_window::exit_callback): Removed declaration. (main_window::confirm_exit_octave): Renamed confirm_shutdown_octave(). * octave-qt-link.cc (octave_qt_link::do_confirm_shutdown): New virtual function definition for initiating GUI shutdown and confirm exit. Worker process goes into sleep mode after issuing cross-thread signal. Return confirmation result. (octave_qt_link::do_exit): Change exit_signal to exit_app_signal. octave-qt-link.h (octave_qt_link::do_confirm_shutdown): New virtual function declaration for qt GUI. (octave_qt_link::mutex): Added. A QMutex for communicating with Qt GUI. (octave_qt_link::waitcondition): Added. For process wait. (octave_qt_link::shutdown_confirmation): Added. Store confirmation result. (octave_qt_link::_shutdown_confirm_result): Added. Confirmation result. (octave_qt_link::exit_signal): Signal renamed exit_app_signal. (octave_qt_link::confirm_shutdown_signal): Added. (octave_qt_link::exit_app_signal): Renamed of exit_signal. octave-link.h (octave_link::confirm_shutdown): Added. Access virtual function do_confirm_shutdown. (octave_link::do_confirm_shutdown): Added. Virtual function for GUI link. toplev.cc (clean_up_and_exit): Change 'retval' to 'status', as header file uses argument name 'status'. (quit): First initiate GUI shutdown via octave_link::confirm_shutdown() and if not confirmed, immediately return thereby avoiding exit sequence.