Delay timer for preview updates

So that preview changes only occur when done making code changes.

Paired with @santiaago on this feature

lib/editor.dart

@@ -29,7 +29,15 @@ class Editor {
    this._ace.setValue(data, -1);
    this._ace.focus();
    this.updatePreview();
    _ace.getSession().on('change',new js.Callback.many((e,a)=>this.updatePreview()));
    _ace.getSession().on('change',new js.Callback.many((e,a)=>this.resetUpdateTimer()));
  }

  Timer _update_timer;
  void resetUpdateTimer() {
    if (_update_timer != null) _update_timer.cancel();

    var wait = new Duration(seconds: 1.5);
    _update_timer = new Timer(wait, ()=> this.updatePreview());
  }

  // worry about waitForAce?
@@ -128,6 +136,11 @@ class Editor {
      ..src = "packages/ice_code_editor/js/ace/ace.js";
    document.head.nodes.add(script);

    // TODO: ask why onKeyDown does not work for arrow keys
    document.onKeyUp.listen((e) {
      if (_update_timer != null) resetUpdateTimer();
    });

    this._waitForAce = new Completer();
    script.onLoad.listen((event) {
      js.context.ace.config.set("workerPath", "packages/ice_code_editor/js/ace");