feat: 1.固定图层缩略图（完成）

2.工具栏新增插槽（完成）
3.loadJSON的元素顺序回发生错误

src/component/Canvas/CanvasEditor/commands/BackgroundCommands.js

@@ -33,7 +33,11 @@ export class CreateBackgroundLayerCommand extends Command {
     const bgObject = this._createBackgroundObject();
 
     // 将背景对象添加到图层中
-    this.backgroundLayer.fabricObject = bgObject;
+    this.backgroundLayer.fabricObject = bgObject.toObject([
+      "id",
+      "layerId",
+      "type",
+    ]);
 
     // 添加图层到最底部
     this.layers.value.push(this.backgroundLayer);