Add unique Node IDs to support base and instantiated scene refactorings

The main goal of this PR is to safeguard when a base or instantiated scene changes (nodes renamed, moved or readded),
that the hierarchy is still maintained and the node and its overridden properties can be preserved.

What it does:
* Implements unique node IDs.
* These IDs act as a fallback to names when saving.
* The IDs are **USED AS A FALLBACK**, so they are just an addition. It should not break any current existing scene.
* If a scene renames or moves a node, inherited or instantiated scenes will no longer lose reference to it.

Unlike the previous approach, this one is intended to be a fallback, only used if the node is not found.
This makes it safer to implement and ensure that, at worst case, we fail to find the node, but nothing breaks.

Author: reduz

core/io/resource_uid.cpp

@@ -351,6 +351,7 @@ void ResourceUID::clear() {
 	unique_ids.clear();
 	changed = false;
 }
+
 void ResourceUID::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("id_to_text", "id"), &ResourceUID::id_to_text);
 	ClassDB::bind_method(D_METHOD("text_to_id", "text_id"), &ResourceUID::text_to_id);

scene/main/node.cpp

@@ -2082,6 +2082,14 @@ Node *Node::find_parent(const String &p_pattern) const {
 	return nullptr;
 }
 
+void Node::set_unique_scene_id(int32_t p_unique_id) {
+	data.unique_scene_id = p_unique_id;
+}
+
+int32_t Node::get_unique_scene_id() const {
+	return data.unique_scene_id;
+}
+
 Window *Node::get_window() const {
 	ERR_THREAD_GUARD_V(nullptr);
 	Viewport *vp = get_viewport();

scene/main/node.h

@@ -132,6 +132,9 @@ class Node : public Object {
 #ifdef DEBUG_ENABLED
 	static SafeNumeric<uint64_t> total_node_count;
 #endif
+	enum {
+		UNIQUE_SCENE_ID_UNASSIGNED = 0
+	};
 
 	void _update_process(bool p_enable, bool p_for_children);
 
@@ -281,6 +284,8 @@ class Node : public Object {
 		mutable bool is_translation_domain_inherited : 1;
 		mutable bool is_translation_domain_dirty : 1;
 
+		int32_t unique_scene_id = UNIQUE_SCENE_ID_UNASSIGNED;
+
 		mutable NodePath *path_cache = nullptr;
 
 	} data;
@@ -527,6 +532,9 @@ class Node : public Object {
 	Node *get_parent() const;
 	Node *find_parent(const String &p_pattern) const;
 
+	void set_unique_scene_id(int32_t p_unique_id);
+	int32_t get_unique_scene_id() const;
+
 	Window *get_window() const;
 	Window *get_non_popup_window() const;
 	Window *get_last_exclusive_window() const;