Implement and document wxDataViewTreeCtrl::IsContainer(), use it in the sample to not add items to non-containers

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62630 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/dataview.h

@@ -2023,6 +2023,11 @@ public:
                               int icon = -1,
                               wxClientData* data = NULL);
 
+    /**
+        Returns true if item is a container.
+    */
+    bool IsContainer( const wxDataViewItem& item );
+    
     /**
         Calls the same method from wxDataViewTreeStore but uses
         an index position in the image list instead of a wxIcon.