QWizard: Black titlebar if Aero disabled after all wizards destroyed

If a QWizard is shown when Aero is enabled, the current visual style is cached in a static member of QVistaHelper. The cached state is updated by QVistaHelper when it receives WM_THEMECHANGED or WM_DWMCOMPOSITIONCHANGED events from Windows.

If all QWizard instances are destroyed, there are no instances of QVistaHelper to receive these notifications and update the cache. If Aero is now disabled, the cached current visual style in QVistaHelper isn't updated. If a wizard is now created and shown, a large black rectangle is shown in the titlebar.