The trick of this UndoManager is to jump over UndoableEdits
of type DocumentEvent.EventType.CHANGE. These are events like
coloring that we don't want to see in undo/redo.
The same as super.editToBeUndone() just that we treat DocumentEvent.EventType.CHANGE
the same way as true == edit.isSignificant().
The method of the super class already seems to be a bit buggy.
The DocumentEvent.EventType.CHANGE fix doesn't remove the bugs but makes it behave