Designer: Modernize

* Remove QLatin1{Char|String}
* Use member initialization
* Use range-for
* Use nullptr

Change-Id: I51c24c0a2066861f59731585f19d61b28c76c0a3
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/designer/settingsmanager.cpp

@@ -67,6 +67,6 @@ QString SettingsManager::addPrefix(const QString &name) const
 {
     QString result = name;
     if (Core::ICore::settings()->group().isEmpty())
-        result.prepend(QLatin1String("Designer"));
+        result.prepend("Designer");
     return result;
 }