Add a "Not Done" button for completed ToDos

It doesn't have to be a button for every row in the Done tab. Instead, it could just involve waiting a few seconds before dismissing a ToDo that's been marked as Done and presenting the button during that transition state. Kind of like the Undo feature in Gmail.

Central issue #18877 (moved)

Final design

For the todos in the Done tab we show a Not done button on hover.