Add Undo to deleted lines and blocks

[Illviljan]

I accidentally removed a Base Type line and wanted to undo it. That didn't work. Does not work for other lines and blocks either.

[ben-wallis]

Adding undo functionality is a massive piece of work since it'd involve rearchitecting the entire workflow to use the command pattern so that actions can be undone. This won't be added in the near future but is something I want to implement eventually.