Updates portuguese strings.xml

As requested by @barbeau , here are the portuguese translations.

I've done them as I understood the XML file, they might be too big for some layouts, I don't know. When the beta version is released I'll do a tour of the app and check if anything is wrong. Either way, it would be great in the future to have some screenshots of the "new" layouts, to better understand the strings context. :)

The License was not translated since I don't think it should be (I haven't found an Apache License written in other language rather than English).

Keep up the good work!
Best regards