Replace lastVisited method in DocInfo with timestamp attribute

This is not only more consistent with the name, uri and
mimeType attributes, but makes adding Zeitgeist support easier.

js/ui/widget.js

@@ -359,7 +359,7 @@ RecentDocsWidget.prototype = {
                 items.push(docInfo);
         }
 
-        items.sort(function (a,b) { return b.lastVisited() - a.lastVisited(); });
+        items.sort(function (a,b) { return b.timestamp - a.timestamp; });
         for (i = 0; i < Math.min(items.length, 5); i++)
             this.addItem(items[i]);
     }