edit_curv does not manage undo/redo correctly

Reported by Clement DAVID (@davidcl)

BUG DESCRIPTION:
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Instead of storing, the last value and use it on undo. The undo/redo management should track changes
in a list and a redo index to manage the redo position.


ERROR LOG:
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None

HOW TO REPRODUCE THE BUG:
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edit_curve()
// click to add some points
// click on Edit->Undo twice

OTHER INFORMATION:
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