QWindowsWindow: Use updated windowFlags for framelessWindow query

Currently the internal flags of QWindow are not up-to-date when querying
them during QWindowsWindow::setFlags with QWindow::flags(). QWindow
updates its flags after QWindowsWindow returns from setFlags. This patch
adds a getter, to query the updated internal state of
Qt::FrameLessWindowHint flag from QWindowsWindow's m_data.flags.

This is needed due to QWindowsWindow is calling SetWindowPos, which
immediately enters the WndProc without access to the newly set flags.

Pick-to: 6.7
Change-Id: I826f29ef608d1e0a4b416e123aa4116587a035a7
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 0e6907d2f0e95a8892df55e075837cc0e81e0f3f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qwindowswindow.h

@@ -347,6 +347,8 @@ public:
     int savedDpi() const { return m_savedDpi; }
     qreal dpiRelativeScale(const UINT dpi) const;
 
+    bool isFrameless() const { return m_data.flags.testFlag(Qt::FramelessWindowHint); }
+
 private:
     inline void show_sys() const;
     inline QWindowsWindowData setWindowFlags_sys(Qt::WindowFlags wt, unsigned flags = 0) const;