css: add a 'hidden' style class

Used for elements that are not supposed to be shown.

https://bugzilla.gnome.org/show_bug.cgi?id=675341

data/theme/gnome-shell.css

@@ -1948,10 +1948,12 @@ StScrollBar StButton#vhandle:hover
     padding-bottom: 8px;
 }
 
-/* intentionally left transparent to avoid dialog changing size */
+.hidden {
+    color: rgba(0,0,0,0);
+}
+
 .prompt-dialog-null-label {
     font-size: 10pt;
-    color: rgba(0,0,0,0);
     padding-bottom: 8px;
 }
 

js/ui/polkitAuthenticationAgent.js

@@ -167,6 +167,7 @@ const AuthenticationDialog = new Lang.Class({
          */
         this._nullMessageLabel = new St.Label({ style_class: 'prompt-dialog-null-label',
                                                 text: 'abc'});
+        this._nullMessageLabel.add_style_class_name('hidden');
         this._nullMessageLabel.clutter_text.ellipsize = Pango.EllipsizeMode.NONE;
         this._nullMessageLabel.clutter_text.line_wrap = true;
         messageBox.add(this._nullMessageLabel);