chatgui: add shutdown diagnostics (poll exit reason, cleanup steps, app.exec return)

tools/chatgui/src/main.cpp

@@ -95,5 +95,7 @@ int main(int argc, char *argv[]) {
     SetConsoleCtrlHandler(console_ctrl_handler, TRUE);
 #endif
 
-    return app.exec();
+    int ret = app.exec();
+    qDebug("main: app.exec() returned %d", ret);
+    return ret;
 }