Implement wxNativeContainerWindow::IsShown() for MSW.

Simply use native ::IsWindowVisible() for IsShown() implementation.

Closes #11503.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62783 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/nativewin.h

@@ -145,6 +145,9 @@ public:
         return false;
     }
 
+#ifdef __WXMSW__
+    virtual bool IsShown() const;
+#endif // __WXMSW__
 
     // this is an implementation detail: called when the native window is
     // destroyed by an outside agency; deletes the C++ object too but can in