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/tipdlg.cpp

@@ -76,7 +76,7 @@ class WXDLLIMPEXP_ADV wxFileTipProvider : public wxTipProvider
 public:
     wxFileTipProvider(const wxString& filename, size_t currentTip);
 
-    virtual wxString GetTip();
+    virtual wxString GetTip() wxOVERRIDE;
 
 private:
     wxTextFile m_textfile;