Bug 389429, add an open property for menus, r=neil

toolkit/content/widgets/menu.xml

@@ -45,7 +45,15 @@
            extends="chrome://global/content/bindings/menu.xml#menuitem-base">
 
     <implementation implements="nsIDOMXULContainerElement">
-    <!-- nsIDOMXULContainerElement interface -->
+      <property name="open" onget="return this.hasAttribute('open');">
+        <setter><![CDATA[
+          this.boxObject.QueryInterface(Components.interfaces.nsIMenuBoxObject)
+              .openMenu(val);
+          return val;
+        ]]></setter>
+      </property>
+
+      <!-- nsIDOMXULContainerElement interface -->
       <method name="appendItem">
         <parameter name="aLabel"/>
         <parameter name="aValue"/>