[welcome] Lookup GeoIP language and pass to language widget

- FIXES #934
 - Whether this is really wanted depends on the distro, and I'm not
   100% convinced the likely tags from Unicode are correct (or it'd
   take a lot more data). In any case, starting Calamares in "NL"
   gets me "nl_NL" as translation; presumably starting it in "BE"
   will get me that as well (what about Les Wallons?)
 - This also shows off that it's a real hack to have so much program
   logic in the *widget* parts of each ViewStep. Longer-term,
   a lot of functionality should go to the ViewStep itself, which
   will then control the UI.

src/modules/welcome/WelcomeViewStep.cpp

@@ -161,5 +161,11 @@ WelcomeViewStep::setCountry( const QString& countryCode )
         return;
     }
     else
-        cDebug() << "Would like to select" << c_l;
+    {
+        int r = CalamaresUtils::Locale::availableTranslations()->find( countryCode );
+        if ( r < 0 )
+            cDebug() << "Unusable country code" << countryCode << "(no suitable translation)";
+        if ( ( r >= 0 ) && m_widget )
+            m_widget->externallySelectedLanguage( r );
+    }
 }