add periodic.

Runtime/ui/window.cs

 
         public abstract Timer run(TimeSpan duration, Action callback, bool periodic = false);
 
+        public Timer periodic(TimeSpan duration, Action callback) {
+            return this.run(duration, callback, true);
+        }
+
         public Timer run(Action callback) {
             return this.run(TimeSpan.Zero, callback);
         }