message: fix qt dpi scaling attribute complaint
  In Ubuntu 20.04, qt will throw a warning "Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created." with Qt version 5.12.8
diff: @@ -562,13 +562,12 @@ int main(int argc, char *argv[])
  Q_INIT_RESOURCE(bitcoin);
  Q_INIT_RESOURCE(bitcoin_locale);
  - BitcoinApplication app(argc, argv);
  #if QT_VERSION > 0x050100
  // Generate high-dpi pixmaps
  QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
  #endif
  #if QT_VERSION >= 0x050600
  - QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
  + QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
  #endif
  #ifdef Q_OS_MAC
  QApplication::setAttribute(Qt::AA_DontShowIconsInMenus);
  @@ -586,6 +585,8 @@ int main(int argc, char *argv[])
  Q_ASSERT(QSslConfiguration::defaultConfiguration().protocol() == QSsl::TlsV1_0OrLater);
  #endif
  + BitcoinApplication app(argc, argv);
  +
  // Register meta types used for QMetaObject::invokeMethod
  qRegisterMetaType< bool* >();
  // Need to pass name here as CAmount is a typedef (see http://qt-project.org/doc/qt-5/qmetatype.html#qRegisterMetaType)
  