Override this to indicate which wxDataViewItem representing the parent
of {\it item} or an invalid wxDataViewItem if {\it item} is the root item.
\membersection{wxDataViewModel::GetValue}\label{wxdataviewmodelgetvalue}
Override this to indicate which wxDataViewItem representing the parent
of {\it item} or an invalid wxDataViewItem if {\it item} is the root item.
\membersection{wxDataViewModel::GetValue}\label{wxdataviewmodelgetvalue}