implemented 3state checkboxes in wxGTK with GTK2

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@28035 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/checkbox.h

@@ -63,6 +63,11 @@ public:
 protected:
     virtual wxSize DoGetBestSize() const;
 
+#ifdef __WXGTK20__
+    void DoSet3StateValue(wxCheckBoxState state);
+    wxCheckBoxState DoGet3StateValue() const;
+#endif
+
 private:
     DECLARE_DYNAMIC_CLASS(wxCheckBox)
 };