调整了一下各种命令的位置，便于其他应用调用

AIStudio.Wpf.DiagramDesigner/ViewModels/SimpleViewModel/MediaItemViewModel.cs

@@ -19,6 +19,21 @@ namespace AIStudio.Wpf.DiagramDesigner
 
         }
 
+        public MediaItemViewModel(IDiagramViewModel parent, string json) : base(parent, json)
+        {
+
+        }
+
+        public override SelectableDesignerItemBase ToXmlObject()
+        {
+            return new MediaDesignerItem(this);
+        }
+
+        public override Type ToXmlType()
+        {
+            return typeof(MediaDesignerItem);
+        }
+
         protected override void Init()
         {
             base.Init();