Export default message handler

[cloned from the QTBUG-388; this would still be a usefull feature to be implemented.]

It would be great if there was a default message handler on all platforms. If this was the case, one could simply do something like:

QtMsgHandler defaultMsgHandler = qInstallMsgHandler(myMsgHandler);

void myMsgHandler(QtMsgType type, const char* msg)
{
defaultMsgHandler(type, msg);
doSomethingElse(type, msg); // for example, write to a log
}