implemented GetBestSize() (TODO: make this available in generic version as well)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35856 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/treectrl.h

@@ -429,6 +429,9 @@ public:
     int GetState(const wxTreeItemId& node);
 
 protected:
+    virtual wxSize DoGetBestSize() const;
+
+    
     // SetImageList helper
     void SetAnyImageList(wxImageList *imageList, int which);