Ensure that SceneTree is initialized and finalized at correct time

[RedwanFox]

SceneTree should be fully initialized before any tree operation with any node and finalized only after all nodes exited tree.

Accounts only for in-tree operations, not constructors. After fix init-deinit order will be:

1. constructors (_init) in unspecified order
2. SceneTree native _initialize
3. SceneTree script _initialize
4. All nodes in-tree lifecycle (_enter_tree ->_ready -> ...-> _exit_tree)
5. SceneTree script _finalize
6. SceneTree native _finalize

Partially fixes https://github.com/godotengine/godot/issues/71695

[akien-mga]

Possibly related to #70771 ? CC @kleonc

[RedwanFox]

Added one more pull request as follow up to this https://github.com/godotengine/godot/pull/72266 It's experimental and is meant for discussion.

[kleonc]

Possibly related to #70771 ? CC @kleonc

@akien-mga Kinda related as both are fixing some aspects of likely not-so-logical initialization order for MainLoop/SceneTree. But I do think both PRs are fine on their own / independent.

[YuriSizov]

Needs a rebase though :)

[RedwanFox]

@RandomShaper Could you also look into this https://github.com/godotengine/godot/pull/70771 ? It would be nice to have both fixes in upstream @YuriSizov done

[YuriSizov]

Thanks! Yeah, we're aiming to merge this for 4.2 dev1.

[YuriSizov]

Thanks!