Delete windows before application on session end in wxMSW

When WM_ENDSESSION was received by the application, the wxApp object
itself was shut down by calling OnExit() on it before all the TLWs were
destroyed, which could be completely unexpected as during normal
shutdown the order of events is exactly the reverse.

In practice, this resulted in crashes in any application whose main
window close event handler or dtor touched wxTheApp in any way (e.g. to
save any configuration in the global wxConfig object destroyed by
wxApp::OnExit()).

See #9590 (sorry for missing the point before, ATS).

include/wx/app.h

@@ -684,6 +684,10 @@ protected:
     // override base class method to use GUI traits
     virtual wxAppTraits *CreateTraits() wxOVERRIDE;
 
+    // Helper method deleting all existing top level windows: this is used
+    // during the application shutdown.
+    void DeleteAllTLWs();
+
 
     // the main top level window (may be NULL)
     wxWindow *m_topWindow;