Add wxOVERRIDE and use it in common and wxOSX code.

Make overriding virtual methods more explicit and enable additional checks
provided by C++11 compilers when "override" is used.

Closes #16100.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76173 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/window_osx.cpp

@@ -156,7 +156,7 @@ public:
 
     virtual ~wxBlindPlateWindow();
 
-    virtual bool AcceptsFocus() const
+    virtual bool AcceptsFocus() const wxOVERRIDE
     {
         return false;
     }