desktop: Migrate to winit 0.30's ApplicationHandler

[Dinnerbone]

This is one of the TODOs from #17401

Needs testing please! Largest change is to ControlFlow (specifically, how often we redraw things). In the previous update model, we'd force it to wait until the next frame time after every event we receive. Now, we conditionally do it at the end of a batch of events.

There's still another pending TODO (make the window later, instead of before event loop) - but that's a larger and messier change.
We can also clean up the ControlFlow usage further by using a callback in egui, but I didn't want to risk more changes than needed.