QDate/QTime/QDateTime use QLocale::system(), which makes them non-reentrant

Some methods of QDate/QTime/QDateTime - most notably the toString methods - are internally using QLocale::system(). QLocale::system() calls the static systemData() method, which in turn can modify global state (in QLocalPrivate::updateSystemPrivate()). This produces race conditions, when e.g. the toString functions are called simultaneously from multiple threads (even on different QDate instances) and caused various random crashes in my project.