windows: Replace Q_OS_WIN with Q_OS_WIN_TODO so we can start porting

Now it builds. This way I can port each Q_OS_WIN one by one and make
sure everything still builds, without risking breaking Linux/macOS.

examples/qtquick/customtitlebar/main.cpp

@@ -36,7 +36,7 @@ CustomViewFactory::~CustomViewFactory() = default;
 
 int main(int argc, char *argv[])
 {
-#ifdef Q_OS_WIN
+#ifdef Q_OS_WIN_TODO
     QGuiApplication::setAttribute(Qt::AA_UseOpenGLES);
 #endif
     QGuiApplication app(argc, argv);