Hide "whats this" buttons on all dialogs on Windows

Since it's totally useless and does nothing at all.

*Requires Qt >= 5.10

src/app/main.cpp

@@ -793,6 +793,9 @@ int main( int argc, char *argv[] )
   QCoreApplication::setOrganizationDomain( QgsApplication::QGIS_ORGANIZATION_DOMAIN );
   QCoreApplication::setApplicationName( QgsApplication::QGIS_APPLICATION_NAME );
   QCoreApplication::setAttribute( Qt::AA_DontShowIconsInMenus, false );
+#if QT_VERSION >= 0x051000
+  QCoreApplication::setAttribute( Qt::AA_DisableWindowContextHelpButton, true );
+#endif
 
   QgsSettings settings;
   if ( configLocalStorageLocation.isEmpty() )