event-manager: Add function to signal valid implementation of GUI dialogs (bug #60980). The event-manager is also enabled when starting with --no-gui-libs. But not all implementations of the event-manager have dialogs (only Qt at the moment which isn't used when starting with --no-gui-libs). Add functions that return if the event-manager has implemented dialogs. * libinterp/corefcn/event-manager.h (interpreter_events::have_dialogs, event_manager::have_dialogs): Add new function "have_dialogs". (event_manager::file_dialog, event_manager::input_dialog, event_manager::list_dialog, event_manager::question_dialog): Guard with "have_dialogs". * libinterp/corefcn/event-manager.cc (F__event_manager_have_dialogs__): Add new function. * libgui/src/qt-interpreter-events.h (qt_interpreter_events::have_dialogs): Add new function. * scripts/gui/inputdlg.m, scripts/gui/listdlg.m, scripts/gui/msgbox.m, scripts/gui/questdlg.m, scripts/gui/uigetdir.m, scripts/gui/uigetfile.m, scripts/gui/uiputfile.m: Check "____event_manager_have_dialogs__" instead of "__event_manager_enabled__". * scripts/gui/private/__get_funcname__.m: Try to use dialog functions specific to the current graphics toolkit if the event-manager doesn't have implemented dialogs.

--- a/libgui/src/qt-interpreter-events.h	Fri Sep 17 12:46:59 2021 -0700
+++ b/libgui/src/qt-interpreter-events.h	Sun Sep 19 15:36:19 2021 +0200
@@ -86,6 +86,8 @@
     void start_gui (bool gui_app = false);
     void close_gui (void);
 
+    bool have_dialogs (void) const { return true; }
+
     std::list<std::string>
     file_dialog (const filter_list& filter, const std::string& title,
                  const std::string& filename, const std::string& pathname,