added 3-state checkboxes (patch 813790)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@23982 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/checkbox.h

@@ -42,7 +42,7 @@ public:
                 const wxValidator& validator = wxDefaultValidator,
                 const wxString& name = wxCheckBoxNameStr);
 
-    virtual void SetValue(bool value);
+    virtual void SetValue(const bool value);
     virtual bool GetValue() const;
 
     virtual void SetLabel(const wxString& label);
@@ -53,6 +53,10 @@ public:
 protected:
     virtual wxSize DoGetBestSize() const;
 
+    virtual void DoSet3StateValue(wxCheckBoxState value);
+
+    virtual wxCheckBoxState DoGet3StateValue() const;
+
 private:
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxCheckBox)
 };