egui: add redo support to Undoer - #3478

Honestly no idea why the labels check is failing. Either that or it means actual github labels which I don't have permission to apply.

Hey @emilk what would it take to get this merged?

Better implementation than #3448

Better how?

Better implementation than #3448

Better how?

AFAIK this implementation just seemed more straightforward to me. I've since forgot the exact reason, though.

Sigh. I literally ran cargo cranky. Not very cranky if it doesn't check formatting.

In my own application, I have a variation of change history that allows you to have multiple redo stacks and switch between them, rather than them being cleared whenever you perform an action. However, that usually requires supporting UI, and isn't always intuitive to people, which is why I don't think it's right for egui to implement that itself. However, TextEdit makes it difficult to replace the default undo and redo actions with it. Not sure if there is an easy solution.