extract (and expand and clean up and document) the header window implementation used inside the generic wxDataViewCtrl in a separate wxHeaderCtrl class which could be reused in (generic) wxListCtrl and, most importantly, wxGrid later

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57093 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/treectrl.h

@@ -242,8 +242,10 @@ private:
     // the common part of all ctors
     void Init();
 
-    // helper functions
+    // helper functions: DoGetItem() doesn't work for hidden virtual root item
+    // while DoGetPossiblyRootItem() does
     bool DoGetItem(wxTreeViewItem *tvItem) const;
+    bool DoGetPossiblyRootItem(wxTreeViewItem *tvItem) const;
     void DoSetItem(wxTreeViewItem *tvItem);
 
     void DoExpand(const wxTreeItemId& item, int flag);