Implement TreeItem.add_child

[YuriSizov]

#46773 implemented some of the requested item management methods, but one thing was amiss — a way to add an unparented TreeItem back to the Tree (or move it to another Tree). The issue was made worse by the fact that we have a TreeItem::remove_child method, but no TreeItem::add_child method. Documentation for remove_child states that the unparented item can be reused, but how?

At the same time, aforementioned PR added move_before/move_after, which actually support moving items between different trees. So the implementation of the add_child method was on the surface. And it was explicitly asked for in godotengine/godot-proposals#3607.

Personally, I thought about using it for some sort of cache in the editor. For example, in the editor help search dialog we could create every TreeItem that we need and then reuse them, instead of recreating them on the fly, constantly. I was working on that, but decided to split Tree improvements for now.

Additional changes in this PR include:

• Better names for the create_child method variables;
• Add a missing (I think) call to _change_tree(nullptr) to the remove_child method;
• Move some methods around to group them logically;
• Small style improvements in the files.

Closes godotengine/godot-proposals#3607.

godot.windows.editor.dev.x86_64_2023-05-24_19-40-37.mp4

[trollodel]

I wanted to add this since the proposal was opened, but I forgot about that.
Good that you added it.

[YuriSizov]

Okay, changes done.