avoid code duplication between the generic and GTK versions, factor the common code out in OnStateChange()

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42264 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/collpaneg.h

@@ -71,6 +71,10 @@ public:
     virtual wxString GetLabel() const
         { return m_strLabel; }
 
+
+    // implementation only, don't use
+    void OnStateChange(const wxSize& sizeNew);
+
 protected: 
     // overridden methods
     virtual wxSize DoGetBestSize() const;