messageTray: Split out addButton to allow consumers to pass a pre-made button

Some consumers may want to construct their buttons specially, so allow them
to do that by adding a new API that takes a button instead of a label.

https://bugzilla.gnome.org/show_bug.cgi?id=710137

js/ui/overview.js

@@ -79,7 +79,7 @@ const ShellInfo = new Lang.Class({
 
         this._undoCallback = undoCallback;
         if (undoCallback) {
-            notification.addButton('system-undo', _("Undo"));
+            notification.addAction('system-undo', _("Undo"));
             notification.connect('action-invoked', Lang.bind(this, this._onUndoClicked));
         }