Mark accomplished todos

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The idea to mark accomplished todos is listed under Things to Improve in the todonotes documentation.

I implemented this idea for my LaTeX projects and created a thread on tex.stackexchange describing the implementation in detail. I hoped to get suggestions on further improvement but have only gotten thumbs-up. Also, I have had no problems with it myself within the last 1,5 years.

Please consider including the code, its parts, or any ideas from the thread linked above into your package, if you find it worthwhile.

Thank you!