Improve Scene Tree editor performance

We now cache the Node*<>TreeItem* mapping in the SceneTreeEditor. This
allows us to make targeted updates to the Tree used to display the scene
tree in the editor.

Previously on almost all changes to the scene tree the editor would
rebuild the entire widget, causing a large number of deallocations an
allocations. We now carefully manipulate the Tree widget in-situ saving
a large number of these allocations.

There is definitely more that could be done, but this is already a
massive improvement.

This fixes #83460