use signal for setting, appending to, and clearing history widget * history-dockwidget.h, history-dockwidget.cc (history_dock_widget::request_history_model_update, history_dock_widget::reset_model, history_dock_widget::update_history_callback): Delete. (history_dock_widget::set_history, history_dock_widget::append_history, history_dock_widget::clear_history): New functions. * main-window.h, main-window.cc (main_window::update_history): Delete. (main_window::construct): Delete connection from _octave_qt_event_listener::update_history_signal to main_window::update_history. Connect _octave_qt_link::set_history_signal to _history_dock_widget::set_history. Connect _octave_qt_link::append_history_signal to _history_dock_widget::append_history. Connect _octave_qt_link::clear_history_signal to _history_dock_widget::clear_history. (main_window::clear_history_callback): Call Fhistory instead of command_history::clear and _history_dock_widget->reset_model. * octave-main-thread.cc (octave_main_thread::run): Don't call octave_link::update_history. * octave-qt-event-listener.h, octave-qt-event-listener.cc (octave_qt_event_listener::update_history): Delete. (octave_qt_event_listener::update_history_signal): Delete. * octave-qt-link.h, octave-qt-link.cc (octave_qt_link::do_update_history): Delete. (octave_qt_link::do_set_history,octave_qt_link::do_append_history, octave_qt_link::do_clear_history): New functions. (octave_qt_link::do_post_input_event): Don't call do_update_history. (set_history_signal, append_history_signal, clear_history_signal): New signals. * octave-event-listener.h (octave_event_listener::update_history): Delete. * input.cc (octave_base_reader::octave_gets): Call command_history::add. * oct-hist.cc (do_history): New option, -c. Call octave_link history functions as needed. (initialize_history): Call octave_link::set_history. (octave_history_write_timestamp): Call octave_link::append_history. (Fedit_history): Document -c. * octave-link.h (octave_link::update_history, octave_link::do_update_history): Delete. (octave_link::set_history, octave_link::append_history, octave_link::clear_history, octave_link::do_set_history, octave_link::do_append_history, octave_link::do_clear_history, octave_link::do_pre_input_event, octave_link::do_post_input_event): New functions.