箭头分离到独立的model中，方便自定义path

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

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Text;
+
+namespace AIStudio.Wpf.DiagramDesigner
+{
+    /// <summary>
+    /// Simple service interface
+    /// </summary>
+    public interface IDiagramServiceProvider : INotifyPropertyChanged
+    {
+        IColorViewModel ColorViewModel
+        {
+            get;
+        }
+        IFontViewModel FontViewModel
+        {
+            get;
+        }
+        IShapeViewModel ShapeViewModel
+        {
+            get;
+        }
+        IDrawModeViewModel DrawModeViewModel
+        {
+            get;
+        }
+        IQuickThemeViewModel QuickThemeViewModel
+        {
+            get;
+        }
+        ILockObjectViewModel LockObjectViewModel
+        {
+            get;
+        }
+        SelectableDesignerItemViewModelBase SelectedItem
+        {
+            get; set;
+        }
+        IColorViewModel CopyDefaultColorViewModel();
+        IFontViewModel CopyDefaultFontViewModel();
+        IShapeViewModel CopyDefaultShapeViewModel();
+    }
+}