Set the UTF-8 codec on pretty much every QTextStream
When checking bug https://bugs.kde.org/show_bug.cgi?id=447730, I found that the gradients saved and loaded text data using QTextStream without setting the codec to UTF-8. That leads to problems when running Krita in a non-UTF8 locale.
This merge request explicitly set the codec to UTF-8 in pretty much every place where we use QTextStream. There might be places where we actually don't want that, so please check...