Function addChild

packages/fairygui/src/display/DisplayObject.ts:302–304

* Add a child display object * 添加子显示对象

(child: DisplayObject)

     * 添加子显示对象
     */
    public addChild(child: DisplayObject): void {
        this.addChildAt(child, this._children.length);
    }

    /**
     * Add a child at specific index