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.
faddd60c40