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/propgrid/propgridiface.cpp

@@ -805,7 +805,7 @@ public:
         m_it.Init( state, flags );
     }
     virtual ~wxPGVIteratorBase_State() { }
-    virtual void Next() { m_it.Next(); }
+    virtual void Next() wxOVERRIDE { m_it.Next(); }
 };
 
 wxPGVIterator wxPropertyGridInterface::GetVIterator( int flags ) const