All undo on step deletion

[NicMcPhee]

At the moment if you delete a step it is lost and gone forever, and given that the - button is right next to the + button it's not that hard to hit the wrong one.

We should probably support a "stack" of undo/redo options so people can fix simple mistakes like that.

I'm not sure if we want a more sophisticated undo/redo system, or even what that would look like. That's probably another issue (or epic), though.