add GetControlWidget() accessor to the generic wxCollapsiblePane

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/collpaneg.h

@@ -68,6 +68,11 @@ public:
 
     virtual bool Layout();
 
+
+    // for the generic collapsible pane only:
+    wxControl* GetControlWidget() const
+        { return m_pButton; }
+
     // implementation only, don't use
     void OnStateChange(const wxSize& sizeNew);