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.

src/qtwidgets/DebugWindow.h

@@ -37,7 +37,7 @@ public:
     explicit DebugWindow(QWidget *parent = nullptr);
 
 private:
-#ifdef Q_OS_WIN
+#ifdef Q_OS_WIN_TODO
     void dumpWindow(QWidget *);
     void dumpWindows();
 #endif