Add a built-in egui debugger

[emilk]

Debugging layout problems and similar in egui is painful.

I'd like to have a mode where egui records events to an event stream containing:

• Shapes being painted
• Ui creation
• Widget placements
• changes to min_rect and max_rect
• custom text log events
• …

This would then be paired with a viewer of these events, where the user can step through them at their own pace and figure out exactly what happened when.

For instance: you hit a special key-combo, and the next egui frame is recorded as an event stream, and a new egui::Window or viewport pops up that shows this recording, with controls for stepping through the events one at the time. Each event would be visualized as rectangles and shapes in a canvas area.