GtkDialog: protect against show_all

The real reason for the problem that the just reverted change
was supposed to address is that testfilechooser uses show_all
to tease out places where composite dialogs don't properly
protect their internals against unintended showing. Well,
lets do that, then.

gtk/resources/ui/gtkdialog.ui

@@ -25,6 +25,7 @@
         <child>
           <object class="GtkBox" id="action_box">
             <property name="visible">True</property>
+            <property name="no-show-all">True</property>
             <property name="can_focus">False</property>
             <style>
               <class name="dialog-action-box"/>