Handle earmarks properly in undo/redo

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Before this commit, undo/redo didn't know anything about earmarks. This can lead to annoyances in real-world usage of the application: Imagine you set some earmarks on a cell and then think some of those numbers should be the real value. So you set the value to be what you want, but shortly afterwards notice that it was nonsense actually. So you undo, but now the earmarks you set before are lost! This is not what undo should be like.