Fix server crash on client disconnect
When a web client disconnects, the associated QWebSocket object is deleted via WebServer::socketDisconnected. So far this happens by immediately calling the destructor (as we release the unique_ptr from the m_clients list).
However, the QWebSocket may still be needed in the Qt event loop at this time, so the event loop will crash on the next iteration. We have to call QObject::deleteLater(), instead.