made it possible to customize wxGenericDirCtrl, such as using checked wxTreeCtrl (patch #1019699)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30249 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/dirctrlg.h

@@ -177,6 +177,7 @@ protected:
                 int image = -1, int selectedImage = -1,
                 wxTreeItemData * data = NULL);
     //void FindChildFiles(wxTreeItemId id, int dirFlags, wxArrayString& filenames);
+    virtual wxTreeCtrl* CreateTreeCtrl(wxWindow *parent, wxWindowID id, const wxPoint& pos, const wxSize& size, long treeStyle);
 
     // Extract description and actual filter from overall filter string
     bool ExtractWildcard(const wxString& filterStr, int n, wxString& filter, wxString& description);