add test sample

Samples/UIWidgetsSamples_2019_4/Assets/Editor/EditorWindowSample/UnityObjectDragging.cs

+using System.Collections.Generic;
+using uiwidgets;
+using Unity.UIWidgets.Editor;
+using Unity.UIWidgets.gestures;
+using Unity.UIWidgets.widgets;
+using UnityEditor;
+using UnityEngine;
+
+namespace UIWidgetsEditorWindowSample
+{
+    public class UnityObjectDraggingWindow : UIWidgetsEditorPanel
+    {
+        [MenuItem("UIWidgets/EditorSample/UnityObjectDragging")]
+        public static void CountDemo()
+        {
+            CreateWindow<UnityObjectDraggingWindow>();
+        }
+
+        protected override void onEnable()
+        {
+            AddFont("Material Icons", new List<string> {"MaterialIcons-Regular.ttf"}, new List<int> {0});
+            AddFont("CupertinoIcons", new List<string> {"CupertinoIcons.ttf"}, new List<int> {0});
+            AddFont("GalleryIcons", new List<string> {"gallery/GalleryIcons.ttf"}, new List<int> {0});
+        }
+
+        protected override void main()
+        {
+            editor_ui_.runEditorApp(new MyApp());
+        }
+        
+        public class MyApp : StatelessWidget
+        {
+            public override Widget build(BuildContext context)
+            {
+                return 
+                    new Container(color: Colors.green, child:
+                    new Center(child: 
+                        new UnityObjectDetector(
+                            child: new Container(color: Colors.blue, width: 200f, height: 200f, 
+                                child: new Center(child: new Text("Drag UnityObject or file to me"))),
+                                onEnter: (details) =>
+                                {
+                                    Debug.Log("enter");
+                                },
+                                onHover: (details =>
+                                {
+                                    Debug.Log("position = " + details.position);
+                                }),
+                                onExit: () =>
+                                {
+                                    Debug.Log("on exit");
+                                },
+                                onRelease: (details) =>
+                                {
+                                    Debug.Log("on release");
+                                }
+                        )
+                        )
+                    );
+            }
+    }
+    }
+}

Samples/UIWidgetsSamples_2019_4/Assets/Editor/EditorWindowSample/UnityObjectDragging.cs.meta

+fileFormatVersion: 2
+guid: 9916a1957385427fa807b2ce6a676522
+timeCreated: 1637912586