Node: Rename child_exited_tree to child_exiting_tree

[akien-mga]

The name was confusing as this signal is emitted around the same time as
tree_exiting and NOTIFICATION_EXIT_TREE, i.e. while the child node is
still in tree.

Fixes #59210.

[akien-mga]

For the record, I don't really see a clear need for an actual child_exited_tree that would be emitted once the child node has fully exited the tree, because at this stage it's a bit late to do any related changes (and to even know what node this was, as it might have been freed from memory if it exited the tree due to a call to free() or queue_free()).

Still, here's a quick workaround if someone really needs this, you can connect to child_exiting_tree and there connect to the child node's tree_exited, e.g. with a lambda. Example code adapted from the MRP in #59210:

extends Node2D

var someNode

func _ready():
	someNode = get_node("Node2D")
	child_exited_tree.connect(onChildExited)
	remove_child(someNode)
	
func onChildExited(node):
	node.tree_exited.connect(
		func():
			# Do stuff. This is a lambda in parent scope.
			print(get_name()) # Root
			print(get_child_count()) # 0
	)

[akien-mga]

Added some more documentation to clarify the order in which the various tree related signals and notifications are received.

[akien-mga]

Cherry-picked for 3.5.