Corrected/added support for column headers with icon and text

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@53147 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/dataview.h

@@ -325,6 +325,10 @@ private:
     // holds the GTK handle
     GtkWidget   *m_column;
     
+    // holds GTK handles for title/bitmap in the header
+    GtkWidget   *m_image;
+    GtkWidget   *m_label;
+    
     // delayed connection to mouse events
     friend class wxDataViewCtrl;
     void OnInternalIdle();