Expose "editable_instance" to scripting + Update C++ API

[revilo]

This PR fixes #20292.

Changes:

1. Exposes the editable_instance flag to scripts:

This will allow easier implementation of undo/redo for turning on/off the "Editable children" option and its related actions in the editor.

2. Modified the C++ interface of editable_instance to simplify its usage and to better reflect how it works today:

	// old interface
	void set_editable_instance(Node *p_node, bool p_editable);
	bool is_editable_instance(const Node *p_node) const;

	// new interface
	void set_editable_instance(bool p_editable);
	bool is_editable_instance() const;

In the previous implementation, the set_editable_instance and is_editable_instance got called on the owner of a node:

	// old usage
	owner->set_editable_instance(child, true);

This was for historical reasons, as initially the editable_children were managed in a list inside the owner. Today the "editable" state is a simple flag in the Node class itself, and therefore the interface should reflect this:

	// new usage
	child->set_editable_instance(true);

3. Updated all calls to set_editable_instance and is_editable_instance throughout the source code.

[revilo]

I've just seen this comment on another PR saying that having "editable_instance" in the scripting API of Node is not wanted:

#20569 (comment)

What is your opinion on the proposed API change of set_editable_instance and is_editable_instance?

[hilfazer]

What is your opinion on the proposed API change of set_editable_instance and is_editable_instance?

They could be in a separate PR.

[revilo]

They could be in a seperate PR.

Ok, will do so when #20292 gets closed.

[hilfazer]

Changes to the API should probably wait until current issues (#46261) caused by the new way of storing editable children info are resolved.

[hilfazer]

While exposing 'editable_children' may happen in the future it'll probably require a system for distinguishing editor-only properties/methods, as Akien mentioned here #20569 (comment).

I suggest not to wait, remove that part and rename the PR accordingly.

[revilo]

Closed since "editable_instance" is currently not supposed to be exposed to scripting.
The API changes will be submitted in a separate PR.