Feature: add auto save on change

[acezard]

• When the user updates a sheet, auto save the change in the .json
• This requires a way to undo changes (right click would be good, but the user must be aware of the existence of this input)
• This creates a problem for the existence of the save button (what's the point of a save action if everything is always saved)
• Auto-save could be on or off depending on user preference, which solves the above issue. This is also a common toggle feature in applications like Excel