Bug 235872 add description property to menuitems

Relanding xul.css part of patch following aviary branch landing

toolkit/content/xul.css

@@ -278,6 +278,10 @@ menuitem.menuitem-iconic {
   -moz-binding: url("chrome://global/content/bindings/menu.xml#menuitem-iconic");
 }
 
+menuitem[description] {
+  -moz-binding: url("chrome://global/content/bindings/menu.xml#menuitem-iconic-desc-noaccel");
+}
+
 menuitem[type="checkbox"],
 menuitem[type="radio"] {
   -moz-binding: url("chrome://global/content/bindings/menu.xml#menuitem-iconic");