Make reimported models reimport their owner.

[SaracenOne]

First PR in an attempt to triage #91211. Does not fix the whole issue, but does address an important part of it.

Changes the behaviour of the scene hot-reload system so that if the scene which needs to be reimported is owned by another instance, reload that instance instead.

The reason for this is the issue uncovered another oversight in the reimport system. The original approach walked the nodes in each editable scene and made a copy of every scene which required reimport and attempted to reconstruct it as accurately as possible, retaining additional nodes and properties edits. The properties are determined by checking with the standard EditorPropertyRevert::get_property_revert_value and PropertyUtils::is_property_value_different methods. However, the problem arises if your reimported scene is part of another instance, and that instance has its own set of property modifications to the reimported scene. These will appear lost upon reimport since those methods are checking the property values relative to the currently edited scenes but will ignore any changes made by their owned instances.

Fortunately, the fix for this seems fairly straightforward: if a reimported scene is owned by an instance other than the edited scene, mark that instance to be reimported instead. The current code will still walk that instance and preserve all the current changes. Other smaller changes are keeping a full HashMap of nodes marked as editable and folded, and a few typo corrections in the comments.

Still in draft for now since this will need to go through more testing, but I'm reasonably confident that this is the right approach. It may also prove to be an optimization for certain large scenes.

[Hilderin]

Real nice works! Another edge case solved for the scene reloading!! Tested it with success.
I added this situation in the MRP I used to test scene reloading. I created the scene container_editable containing the blender asset, activated editable children and added this scene into simple_inherited without this PR, the modifications in ContainerEditable where lost in simple_inherited on reload and the foldable were resetted in ContainerEditable.
test-godot-blender-reimport-missing-node-v7.zip

Note: If you test the MRP from #91122, it's still broken because the skeleton is now considered modified in the scene. But if you create another scene exactly like 'actor_x', I was not able to reproduce the problem where the skeleton get broken when I modify it in blender and reexport the gltf.

[Hilderin]

Sound good!! Nice work!

[SaracenOne]

@AThousandShips Fixed!

[akien-mga]

Thanks!

[WhalesState]

void update_instance_resource(String p_path, Ref<PackedScene> p_packed_scene); should also be removed from packed_scene.h in the same commit, just in case this commit was reverted for any reason.