#170 [.textw edit undo] return ranges of modified characters

[Francois VOGEL]

I would like the text widget command edit undo to return
as a result the range(s) of characters in the widget that
were modified (impacted) by the undo operation.

This would allow to apply special processing to these
ranges, e.g. colorize these chars, flash them, and so on.

Same request for redo, of course.

Thanks,
Francois