qt: avoid hard-coding font names

They may not contain all necessary characters for a language

src/qt/splashscreen.cpp

@@ -42,7 +42,7 @@ SplashScreen::SplashScreen(Qt::WindowFlags f, const NetworkStyle *networkStyle)
     QString copyrightText   = QChar(0xA9)+QString(" 2009-%1 ").arg(COPYRIGHT_YEAR) + QString(tr("The Bitcoin Core developers"));
     QString titleAddText    = networkStyle->getTitleAddText();
 
-    QString font            = "Arial";
+    QString font            = QApplication::font().toString();
 
     // create a bitmap according to device pixelratio
     QSize splashSize(480*devicePixelRatio,320*devicePixelRatio);