箭头分离到独立的model中，方便自定义path

AIStudio.Wpf.DiagramDesigner/ViewModels/AdditionViewModel/Interface/IShapeViewModel.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Text;
+
+namespace AIStudio.Wpf.DiagramDesigner
+{
+    public interface IShapeViewModel
+    {
+        ILinkMarker SourceMarker
+        {
+            get; set;
+        }
+
+        ILinkMarker SinkMarker
+        {
+            get; set;
+        }
+
+        event PropertyChangedEventHandler PropertyChanged;
+    }
+
+
+}