make undo button and menu item work for command window * main-window.h, main-window.cc (main_window::handle_undo_request, main_window::command_window_undo_callback): New functions. (main_window::construct_edit_menu): Connect _undo_action::triggered to main_window::handle_undo_request. * cmd-edit.h, cmd-edit.cc (command_window::redisplay, command_window::do_redisplay, command_window::undo, command_window::do_undo, gnu_readline::do_redisplay, gnu_readline::do_undo): New functions. * oct-rl-edit.h, oct-rl-edit.c (octave_rl_do_undo): New function.

gnulib-hg = gnulib-hg