#161 Selection doesn\'t change on Redo operation

[Victor Wagner]

When Redo operation is invoked in text widget with
selection,
insertion cursor moves to the redone text, but if there
is a selection which was created after last Undo, it
persists.
This can be misleading for users which thinks that text
typed with active selection should replace selection.

If selection changes would be saved in Undo stack, it
would be more consistent with other (non-Tk ) applications.

[Vince Darley]

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Certainly improvements to the undo mechanism would be good
-- any contributions appreciated.