Update the Polish translation
These patches aim to cleanup and update the Polish translation of Gajim. The “cleanup” part includes replacing a few CRLFs with LFs, using proper Unicode characters instead of simplified ASCII counterparts (where applicable), and deleting old (commented out) messages. By “update” I mean fixing all the fuzzy messages, adding other missing messages, and improving some of the existing ones. I hope I will be able to work on the translation further in the future.