new_editor: support between-row hierarchy drag drop

new_editor/include/XCEditor/Collections/UIEditorTreeView.h

@@ -164,6 +164,14 @@ void AppendUIEditorTreeView(
 
 namespace XCEngine::UI::Editor {
 
+enum class UIEditorTreeViewDropPreviewPlacement : std::uint8_t {
+    None = 0,
+    OnItem,
+    BeforeItem,
+    AfterItem,
+    Root
+};
+
 bool HasUIEditorTreeViewValidBounds(
     const ::XCEngine::UI::UIRect& bounds);
 
@@ -200,6 +208,17 @@ void UpdateUIEditorTreeViewInlineRenameSession(
     const ::XCEngine::UI::UIRect& bounds,
     const std::vector<::XCEngine::UI::UIInputEvent>& inputEvents);
 
+void AppendUIEditorTreeViewDropPreview(
+    ::XCEngine::UI::UIDrawList& drawList,
+    const Widgets::UIEditorTreeViewLayout& layout,
+    const std::vector<Widgets::UIEditorTreeViewItem>& items,
+    bool active,
+    UIEditorTreeViewDropPreviewPlacement placement,
+    std::string_view dropTargetItemId,
+    const ::XCEngine::UI::UIColor& previewColor,
+    float borderThickness = 1.0f,
+    float cornerRounding = 0.0f);
+
 void AppendUIEditorTreeViewDropPreview(
     ::XCEngine::UI::UIDrawList& drawList,
     const Widgets::UIEditorTreeViewLayout& layout,