Add thread support to CreateDialog

[adamscott]

Adds thread support to CreateDialog. This removes will remove any engine-wide lag when searching in the CreateDialog.

Capture.video.du.2024-02-28.15-47-45.webm

(it has some glitches, see the note below about "Draft")

Notes

• This takes into account THREADS_ENABLED, so it should work as well on builds without threads support.

Draft

• This is in an early stage, so a lot will change before it being ready for production.
• Will need to refactor a lot the code, as it is pretty ugly as of 2024-02-28.

Fixes

• Fixes Script classes make the CreateDialog search filter prohibitively slow #27333
• Fixes Slow Create Resource Dialog in a project with a lot of class_names #34205

[adamscott]

So it shows some issues right now with 1000 classes, I'll investigate why the threading doesn't help.

[RandomShaper]

This sort of busy loop is detrimental to performance. I'd suggest you to use a semaphore or a condition variable instead.

[RandomShaper]

The trend is to have as many threaded tasks as possible in the WorkerThreadPool. I'd suggest using it instead of spawning bespoke threads. Besides the reason of having all the threaded work flowing through a single component that may do more clever scheduling in the future, bear in mind that creating a thread is not guaranteed to happen quickly. It's indeed somewhat slow on Windows.

[adamscott]

Closing. I'll investigate more the issue.

[Maran23]

One note here: I did some small but still somewhat noticable improvements for the CreateDialog here #86447
This may or may not be at least a quick win to improve the performance a little bit.
Your Debouncing PR as well as improving the rendering of the Tree, maybe even Thread support are also good ideas to improve this further!