Fix native GTK2 dialogs to receive focus

Task-number: QTBUG-41406
Change-Id: I00f317e5ef62f63c37e9610e1f8ea00af2740b99
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/plugins/platformthemes/gtk2/qgtk2dialoghelpers.cpp

@@ -124,6 +124,7 @@ bool QGtk2Dialog::show(Qt::WindowFlags flags, Qt::WindowModality modality, QWind
     }
 
     gtk_widget_show(gtkWidget);
+    gdk_window_focus(gtkWidget->window, 0);
     return true;
 }