initial fix: enable unpacking GLTF models multiple levels

This is a quick initial fix; Not ready for merge yet

This fixes one major issue with the feature. Previously, you couldn't break apart a GLTF model more than once, as it would just create duplicates of the thing you were trying to break apart

I don't know if the feature works for all cases of GLTF transform hierarchies

Assets/Scripts/Widgets/ModelWidget.cs

@@ -333,7 +333,18 @@ public void SyncHierarchyToSubtree(string previousSubtree = null)
             string subpathToTraverse;
             if (!string.IsNullOrEmpty(previousSubtree))
             {
-                subpathToTraverse = m_Subtree.Substring(previousSubtree.Length);
+
+                // example case:
+                //      previousSubtree = CarBody/Floor
+                //      m_Subtree = CarBody/Floor/Wheel1
+                //      subpathToTraverse should be Floor/Wheel1
+
+                // Floor
+                string lastLevel = previousSubtree.Split("/")[^1];
+
+                int startIndex = previousSubtree.Length - (lastLevel.Length + "/".Length);
+
+                subpathToTraverse = m_Subtree.Substring(startIndex);
             }
             else
             {