Correction and update of Polish translation

Well, the Polish translation is terrible. A lot of strings are untranslated and the UI seems inconsistent. I've corrected many of the current phrases (especially, I've fixed the notation of words to not to use the awful PascalCase anymore) and added messages from newer application releases.

I am going to maintain the translation. Jeśli znalazłeś błąd, zgłoś go.