Remove some run-time dependencies on X11 backend with GTK3

As with Wayland, the X11 backend may not be available at run-time,
so types defined in it can't be used

src/gtk/window.cpp

@@ -1020,7 +1020,11 @@ wxTranslateGTKKeyEventToWx(wxKeyEvent& event,
             }
 
 #ifdef GDK_WINDOWING_X11
-            if (GDK_IS_X11_DISPLAY(gdk_window_get_display(gdk_event->window)))
+#ifdef __WXGTK3__
+            if (strcmp("GdkX11Window", g_type_name(G_TYPE_FROM_INSTANCE(gdk_event->window))) == 0)
+#else
+            if (true)
+#endif
             {
                 // we want to always get the same key code when the same key is
                 // pressed regardless of the state of the modifiers, i.e. on a