Provide localized versions of QLocale::languageToString() and QLocale::countryToString()

The current static methods languageToString() and countryToString() return the english name of the given language/country. However, localized applications may want to display also localized names of countries and languages. One example for such an application is Linguist, which displays the target language of a translation, and is translated in 5 languages.

Since Qt officially supports 5 languages, Qt should provide localized names for these languages. One way would be to add non-static methods languageToString(), and countryToString(), which return localized names if available.