Make qMessageFormatString public

We want to request to make qMessageFormatString public. So we can use it in our own QtMessageHandler.

We implemented a QtMessageHandler to grab logs to write it to a file and into a backlog memory. After that we call the default QtMessageHandler to use the default logging. But we want the same logging output that would be created by qMessageFormatString and the QT_MESSAGE_PATTERN.

The best way would to add an interface to add special logging outputs like the android_logger and journald. The easiest way is to make qMessageFormatString visible to public API.