Update QtWebApp to 1.8.3

nullptr redefinition for old __cplusplus versions has been removed, the rest is vanilla

third_party/QtWebApp/httpserver/httpconnectionhandler.h

@@ -20,7 +20,7 @@
 namespace stefanfrings {
 
 /** Alias type definition, for compatibility to different Qt versions */
-#if QT_VERSION >= 0x050000
+#if QT_VERSION >= QT_VERSION_CHECK(5, 0, 0)
     typedef qintptr tSocketDescriptor;
 #else
     typedef int tSocketDescriptor;