Make Node's add_child() and remove_child() methods virtual

[OptimalFailure]

While working on a plugin for a custom control I ran into inconsistency overriding add_child() to detect children, only to discover that it is not supported and I should not have been able to override it in the first place.
This is not the only way to detect children. While you can detect adding (and removing) children with the "child_entered_tree" (and "child__exiting_tree") signals, temporarily adding/removing a node from the active scene tree will trigger these signals, potentially causing additional overhead. Additionally, a node must be active in the scene tree to emit these signals, meaning the parent will not register the addition of a new child until it is loaded in the scene tree, which makes constructing elements with code more challenging.
For these reasons I suggest exposing add_child() and remove_child() as virtual methods, so custom nodes that manage their children have a straightforward way of tracking additions and removals.
I would appreciate any feedback on this matter!

[AThousandShips]

Closing in favor of:

• Expose Node's add_child() and remove_child() as virtual methods #12766