Set app translation domain before first i18n call

Call here done inside of QApplication subclass constructor code,
so setup has to be done there as well.

Fixes error about empty domain with "Documentation to open"

application.cpp

@@ -27,6 +27,8 @@ Application::Application(int& argc, char** argv)
 {
   setAttribute(Qt::AA_UseHighDpiPixmaps);
 
+  KLocalizedString::setApplicationDomain("khelpcenter5");
+
   mCmdParser.addPositionalArgument( QStringLiteral("url"), i18n( "Documentation to open" ) );
 
   // allow redirecting from internal pages to known protocols
@@ -110,7 +112,6 @@ void Application::open(const QList<QUrl>& urls)
 int main( int argc, char **argv )
 {
   KHC::Application app(argc, argv);
-  KLocalizedString::setApplicationDomain("khelpcenter5");
   KAboutData aboutData( QStringLiteral("khelpcenter"), i18n("Help Center"),
                         QStringLiteral(PROJECT_VERSION),
                         i18n("Help Center"),