Tree refactor

[KoBeWi]

Closes https://github.com/godotengine/godot-proposals/issues/4579

Here's the progress so far:

var item = tree.create_item()
item.get_cell(0).text = "hello"

item = tree.create_item(item)
item.get_cell(0).text = "world"

For now I created class Tree2 to make it easier to compile and test. I'm just copy-pasting the code from the old Tree to the new classes and adjusting it to the new API. Half of the code is commented-out and I'm slowly implementing the missing parts. So far it can apparently draw text items, so there's some hope that this refactor can be finished.

[KoBeWi]

Status update: the progress was very slow, because I don't work on this consistently and recently I've been thinking about abandoning it. Originally I thought that I could just move the code around and then do a cleanup, and while this proved to be possible (based on the current progress), I haven't even touched 5% of the class. It's just ultra time-consuming and I'd rather spend the time on something else. It could've been much easier to just rewrite the class from scratch (similar to what happened to Tweens), but unfortunately I don't know the Tree enough to do that while preserving all of the important functionality.

I'll leave the PR open for now, feel free to take over. Also I don't want to block anyone's work. I've seen some comments mentioning this refactor; just ignore it, as it's uncertain if it even gets finished.