Concise split container titling combining window (class_instance) types for user friendly display

## I'm submitting a…

[ ] Bug

[ ] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

[X] Enhancement

## Current Behavior
Current split con titles, while technically a correct representation of child windows, are not concise or or easy for users to mentally process child window information.

_Please note I'm referring to split container title formatting and not window title formatting. The former is not configurable like window titles are._

## Expected Behavior
Display split con tree information in a more concise and user-friendly format (or an option to do so).

I've implemented what I believe to be an appropriate split con title format in my fork's rewrite of the `con_get_tree_representation` function - see branch [concise-con_get_tree_representation-rewrite](https://github.com/i3/i3/compare/stable...jtaala:concise-con_get_tree_representation-rewrite)).

You can see the difference compared to the current i3 split container title formatting in the two images below (the first is with my fork's changes):

compare this to the current split con titles:

Note I'm just putting this up to see what others think or if there is interest in providing this type of enhancement in base i3wm. If there is, I'd be happy to have the current split con title formatting as default and provide the ability to turn on concise con split titling via an option etc.

If not, I'm happy to continue supporting this enhancement via my fork.

## Environment

Output of `i3 --moreversion 2>&-`:

Binary i3 version:  4.20.1-3-g7c015564 © 2009 Michael Stapelberg and contributors

Running i3 version: 4.20.1-3-g7c015564 (pid 1529893)

Loaded i3 config:

  /home/jay/.i3/config (main) (last modified: Fri 31 Dec 2021 15:31:16, 13256 seconds ago)


The i3 binary you just called: /usr/local/bin/i3

The i3 binary you are running: i3-with-al/bin/i3