Big refactoring: rewrite core `TreeNode` mutable double-linked tree data structure to immutable single-linked persistent tree

[nikitabobko]

TreeNode is a core data structure of AeroSpace and unfortunately it's mutable.

The proposal is to rewrite it to immutable single-linked persistent tree.

Motivation:

1. Stability. The current logic for binding two TreeNode is fragile and sometimes leads to hard to debug crashes: AeroSpace crash when open Xcode windows #706
2. Performance. Once we make AeroSpace multi-threaded to handle hanging apps (Delay/freeze when switching focus & workspace #131), instead of "drawing" the layout on main thread and sending requests to app-threads, it makes sense to pass the tree data structure to each app-thread so that it could draw its part of the tree, which effectively makes the drawing multi-threaded by number of apps. Multi-threading drawing will help to reduce threads context switching for cases when apps have several visible windows. Though the tree has to be traversed multiple times in multi-threaded drawing approach. Precise performance measurements needs to be done.
3. It makes it possible to take snapshots of the tree on its every modification before windows start "dying out" when the screen is locked. It should help to fix this issue: Windows may randomly jump to the focused workspace #1216
4. Features like aerospace undo become possible (though, it may be too far fetched, who would need such a feature?)

Related: #1257

[nikitabobko]

Note to myself: after this issue is fixed, please review all the usages of die and dieT functions to make sure that they are not abused

[zhuoqun-chen]

Hi @nikitabobko, I'm wondering if #726 also related to this meta-issue, I sometimes encounter the same situation when I have many chrome windows and vscode windows.

[nikitabobko]

@zhuoqun-chen no, they are not related. I've answered in #726