longpressup support

Runtime/gestures/long_press.cs

         Offset _longPressOrigin;
 
         TimeSpan? _longPressStartTimestamp;
-        
+
         public GestureLongPressDragStartCallback onLongPressStart;
 
         public GestureLongPressDragUpdateCallback onLongPressDragUpdate;

Runtime/widgets/gesture_detector.cs

                         throw new UIWidgetsError(
                             "Incorrect GestureDetector arguments.\n" +
                             "Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer."
-                            );
+                        );
-                    
+
                     string recognizer = havePan ? "pan" : "scale";
                     if (haveVerticalDrag && haveHorizontalDrag) {
                         throw new UIWidgetsError(
                         );
                     }
                 }
-                
+
                 if (haveLongPress && haveLongPressDrag) {
                     throw new UIWidgetsError(
                         "Incorrect GestureDetector arguments.\n" +

Samples/UIWidgetSample/LongPressSample.cs

-using Unity.UIWidgets.animation;
-using Unity.UIWidgets.engine;
-    public class LongPressSample: UIWidgetsSamplePanel {
-        
-        protected override Widget createWidget()  {
+    public class LongPressSample : UIWidgetsSamplePanel {
+        protected override Widget createWidget() {
             return new WidgetsApp(
                 home: new LongPressSampleWidget(),
                 pageRouteBuilder: this.pageRouteBuilder);
-    public class LongPressSampleWidget: StatefulWidget {
+    public class LongPressSampleWidget : StatefulWidget {
         public override State createState() {
             return new _LongPressSampleWidgetState();
         }
         public override Widget build(BuildContext context) {
             return new GestureDetector(
-                onLongPressDragStart: (value) => {
-                    Debug.Log($"Long Press Drag Start: {value}");
-                },
-                onLongPressDragUpdate: (value) => {
-                    Debug.Log($"Long Press Drag Update: {value}");
-                },
-                onLongPressDragUp: (value) => {
-                    Debug.Log($"Long Press Drag Up: {value}");
-                },
+                onLongPressDragStart: (value) => { Debug.Log($"Long Press Drag Start: {value}"); },
+                onLongPressDragUpdate: (value) => { Debug.Log($"Long Press Drag Update: {value}"); },
+                onLongPressDragUp: (value) => { Debug.Log($"Long Press Drag Up: {value}"); },
+                onLongPressUp: () => { Debug.Log($"Long Press Up"); },
+                onLongPress: () => { Debug.Log($"Long Press"); },
                 child: new Center(
                     child: new Container(
                         width: 200,