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/generic/dirctrlg.cpp

@@ -1529,8 +1529,8 @@ class wxFileIconsTableModule: public wxModule
 DECLARE_DYNAMIC_CLASS(wxFileIconsTableModule)
 public:
     wxFileIconsTableModule() {}
-    bool OnInit() { wxTheFileIconsTable = new wxFileIconsTable; return true; }
-    void OnExit()
+    bool OnInit() wxOVERRIDE { wxTheFileIconsTable = new wxFileIconsTable; return true; }
+    void OnExit() wxOVERRIDE
     {
         wxDELETE(wxTheFileIconsTable);
     }