feat: Add files to the chat asynchronously (GUI)

[SignedAdam]

Added asynchronous file addition to prevent UI freezing when adding multiple files.

I have a large codebase and this can take a few seconds. I find myself writing the prompt outside of aider to not have to wait.

[jbellis]

Good idea. But why is adding a file slow? Can we fix that instead?

[fry69]

Adding so many or so big files that aider feels slow seems to be a bad idea in the first place IMHO. Not sure if we should fix or optimize for that.

See also -> https://aider.chat/docs/faq.html#how-can-i-add-all-the-files-to-the-chat

Maybe related #1587

[SignedAdam]

since there is more interest in the UI now, I can look into it when I have some time 👍

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