Add a custom logging demo

[Calinou]

• Demo counterpart of Add logging documentation godot-docs#11266.
• Follow-up to Add Logger and ability to print and log script backtraces godot#91006.

This demo showcases a custom logger implementation, which runs in parallel to the built-in logging facilities (including file logging). The custom logger displays all messages printed by the engine in an in-game console.

Preview

[mihe]

CustomLogger is not taking into account that _log_error and _log_message can be called from non-main threads (and multiple threads at the same time) so will most likely end up with a race condition when appending to the RichTextLabel, or perhaps tripping some thread guard error somewhere.

I guess deferring the append to the main thread would be the best option in this case, ideally with some comment explaining the thread-safety gotcha.

EDIT: It might be a good idea to add a OS.remove_logger call somewhere in there as well, just to show that it exists, since the user might "leak" stuff otherwise.

[Calinou]

I guess deferring the append to the main thread would be the best option in this case

That makes sense, but I'm not sure how to do it in GDScript. Is it just a matter of using call_deferred()?

[mihe]

Is it just a matter of using call_deferred()?

Yep!

[Calinou]

call_deferred() is now used, and I've made use of OS.remove_logger() too. I've added a comment at the top of the CustomLogger class to emphasize the importance of thread safety.