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.

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();