重写了节点的view、viewmodel关系，实现了对画布元素的选取功能，重构了底层依赖，添加了对net .Framework4.6.1以上的Framework类库支持

Library.Core/Flow/NodeModel/SingleFlipflopNode.cs

@@ -0,0 +1,12 @@
+namespace Serein.Flow.NodeModel
+{
+
+    public class SingleFlipflopNode : NodeBase
+    {
+        public override object Execute(DynamicContext context)
+        {
+            throw new NotImplementedException("无法以非await/async的形式调用触发器");
+        }
+
+    }
+}