Add undo button

[jonko0493]

It would be very nice to have one.

[jonko0493]

Could potentially use something like this to do this.

[jonko0493]

Since we're refactoring everything as part of #371, this is probably a good time to think about implementing something like this.

[jonko0493]

We need to think more about what this even means. In the script editor this makes more sense, but some of the other editors don't really lend themselves well to undo for performance reasons, I think. Unless there's something clever I'm missing.

[jonko0493]

That being said, the linked ReactiveHistory project directly integrates with Avalonia, so it's definitely worth looking at.