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/common/dynload.cpp

@@ -51,14 +51,14 @@ public:
     wxPluginLibraryModule() { }
 
     // TODO: create ms_classes on demand, why always preallocate it?
-    virtual bool OnInit()
+    virtual bool OnInit() wxOVERRIDE
     {
         wxPluginLibrary::ms_classes = new wxDLImports;
         wxPluginManager::CreateManifest();
         return true;
     }
 
-    virtual void OnExit()
+    virtual void OnExit() wxOVERRIDE
     {
         wxDELETE(wxPluginLibrary::ms_classes);
         wxPluginManager::ClearManifest();