Stream generate response to UI

[wwwillchen]

• Streams token to the UI while generating code for UI - this makes the UX a bit more pleasant since you can start reviewing the code right away.
• Switches the final dialog for approving the code changes to use unified diff view to match the intermediate UI a bit more (there's a slightly jarring scroll reset, but it seems unavoidable given that even if we reused the same dialog, it might not be correct to use the current scroll position).