Async support in Ui

I use async extensively in my egui apps, but it's a hassle separating async out because the Widget trait is not async-compatible.

Any plans to better support async in Ui? For example, by asyncifying the Widget trait?

The issue names no files, tests, or entry points. Start by reviewing the Widget trait and Ui APIs to understand the current synchronous boundary. Done would require an agreed design and implementation scope for async support.

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