当 CanStartDrag 返回 true 时调用此函数。
重载此函数可设置全局 DragAndDrop 状态以及可拖放的项。如果需要，使用 FindRows 将拖动的 ID 转换为 TreeViewItem。
按以下顺序控制 TreeView 拖放：
1) 重载 CanStartDrag 以拖动 TreeViewItem。
2) 重载 SetupDragAndDrop 以设置要拖动的 TreeViewItem。
3) 重载 HandleDragAndDrop 以控制 TreeView 的拖放行为。
另请参阅：DragAndDrop。
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