Cleans up the voting action

 - Fixes displaying of poll result in widget
 - Gives an error message if user has already voted
 - Flattens the action structure
'poll:alreadyvoted' => "Entschuldigung: Du kannst leider nur einmal abstimmen.",

I'm always using the informal "Du" instead of the fomal "Sie" everywhere in my German translations. The above translations is also more in line with the translations of the other error messages.

Do you plan to clean up any other files? If yes, I would wait until you've finished before merging your PRs in one go.