Manually set someting as 'DONE' or 'TODO'

Right now, if you want to complete a Todo you have to do some action OR you need to click Done in the Todos view.

That is not a nice workflow if you're coming from email, deep in a thread.

I propose we add a button / link somewhere to mark something either as Done or Todo. Not sure where / what would be easiest without overloading the UI.

@skyruler you have any ideas about the flow and placement of such a button?