Qt's user IDN whitelist code is not thread-safe

Qt allows user to specify custom IDN whitelist using QUrl::setIdnWhitelist(). The list is then stored in a global variable that is accessed without any locking.

The documentation should either specify that this function should only be called before any other thread tries to use the list or the implementation should be made thread safe. I prefer the later because it is hard for a developer to see where the list may be used.