Make qwineventnotifier_p.h (QWinEventNotifier) a public header

QWinEventNotifier is an essential class if you're using native Windows Overlapped IO and need to convert it to Qt signals. However the header is marked private (QtCore/private/qwineventnotifier_p.h). Despite this, some projects that need to use native windows IO use it anyway because it's not feasible to do it any other way. For example qextserialport uses it to convert the native WaitCommEvent to a QIODevice::readyRead() signal. See here: http://code.google.com/p/qextserialport/source/browse/src/qextserialport.h#138 . The Symbian-related plugins for Qt Creator also need to use qwineventnotifier_p.h.

I'd like QWinEventNotifier to be made public in 4.8 so people don't have to rely on private headers that may or may not be exported in their SDK, and so Qt Creator can continue to build cleanly without needing private headers.