Add questdlg dialog function and fix variety of dialog bugs. * octave-qt-link.cc (octave_qt_link::do_message_dialog): Change initializer from QString to QString::fromStdString(dlg). (octave_qt_link::do_question_dialog): New function. Make all buttons AcceptRole. Signal dialog creation. Go to wait state. Get dialog button result when awakened. (octave_qt_link::do_list_dialog): Change std string 'prompt_string' to std list of std string 'prompt'. Use make_qstring_list(prompt). (octave_qt_link::do_input_dialog): Change 'nr' and 'nc' to std list of float. Use QFloatList::fromStdList(nc). * octave-qt-link.h (octave_qt_link : QObject, public octave_link): Add member do_question_dialog. (octave_qt_link::do_list_dialog): Use std list of std string for prompt. (octave_qt_link::do_input_dialog): Change 'nr' and 'nc' to std list of float. * octave-link.cc (__octave_link_question_dialog__): New builtin. Translate args to inputs of octave_link::question_dialog function. (__octave_link_list_dialog__): Retain prompt string cell list as a std list of std string. (__octave_link_input_dialog__): Make 'nr' and 'nc' std list of float. * octave-link.h (octave_link::question_dialog, octave_link::do_question_dialog): Add. (octave_link::list_dialog, octave_link::do_list_dialog): Change prompt to std list of std string. (octave_link::input_dialog, octave_link::do_input_dialog): Change 'nr' and 'nc' to std list of float. * errordlg.m (errordlg): Make retval return variable. Add "error" icon as message_dialog input. * helpdlg.m (helpdlg): Make retval return variable. Add "help" icon as message_dialog input. * warndlg.m (warndlg): Add "warn" icon as message_dialog input. * message_dialog.m (message_dialog): Define default icon "none". Use dlg variable with icon variable as input to __octave_link_message_dialog__. Use __octave_link_enabled__ rather than return value. * inputdlg.m (inputdlg): Round up rowscols to integer. Replace iscell(cstr) with __octave_link_enabled__. * listdlg.m (listdlg): Correct indentation. Change default selmode from "multiple" to "Multiple". Make default prompt {}. Check valid selmode entry. Use __octave_link_enabled__ rather than ok return value. Remove FIXME note. Use Rich Text in prompt of demo. Add demo using ListSize. * questdlg.m (questdlg): Add case 0 to switch. Add error message for default button not matching options. In all cases, check that default button matches an button option. Add __octave_link_question_dialog__ call conditioned under __octave_link_enabled__. Add three demos. * dialog.cc (QUIWidgetCreator::dialog_finished): Remove. (QUIWidgetCreator::dialog_button_clicked): Test that button pointer is nonzero. Move dialog_result assignment and waitcondition.wakeAll here. (MessageDialog::MessageDialog): If title "", change to " ". Reverse order that buttons are installed. Remove signal 'finished' connection. (ListDialog::ListDialog): Change prompt from QString to QStringList. Make list items noneditable. Add Rich Text support to prompt, place RT <br> between prompt list lines. Use label for prompt. Disable select_all button if not "Multiple". Remove horizontal group box. If title "", change to " ". (InputDialog::InputDialog): Change 'nr' and 'nc' to QFloatList. Define pre-processor variables, then undefine. If title "", change to " ". * dialog.h (QUIWidgetCreator::signal_listviewQUI, WidgetCreator::create_listview): Change QString 'prompt_string' to QStringList 'prompt'. (QUIWidgetCreator::signal_inputlayout, QUIWidgetCreator::create_inputlayout): Use QFloatList for 'nr' and 'nc'. (QUIWidgetCreator::dialog_finished): Remove. (MessageDialog::closeEvent): New override, emitting buttonClicked(0). (ListDialog::ListDialog): Change QString 'prompt_string' to QStringList 'prompt'. (InputDialog::InputDialog): Use QFloatList for 'nr' and 'nc'. * main_window.cc, main_window.h: Make function call changes that match dialog.h changes. (main_window::find_files_finished): Remove unused 'button' variable.