Add shortcuts for the combobox (select all, copy, cut, paste)

* It is the same as the code in the implementation of the wxTextCtrl, I
  guess this could be extracted in a common function, but I don't know
  where to put it.

include/wx/osx/combobox.h

@@ -130,11 +130,17 @@ protected:
 
     virtual void EnableTextChangedEvents(bool enable) wxOVERRIDE;
 
+    // callbacks
+    void OnKeyDown(wxKeyEvent& event); // Process clipboard shortcuts
+
     // the subcontrols
     wxComboBoxText*     m_text;
     wxComboBoxChoice*   m_choice;
 
     wxComboBoxDataArray m_datas;
+
+private:
+    wxDECLARE_EVENT_TABLE();
 };
 
 #endif // _WX_COMBOBOX_H_