Use available space better in wxDataViewCtrl date renderer in wxOSX.

Try to show as much useful information as possible for the available width,
notably show the time fully, including seconds, which was never done before.

Also add a date column to the dataview sample to allow seeing how this works
in practice.

See #16640.

samples/dataview/dataview.cpp

@@ -640,6 +640,8 @@ void MyFrame::BuildDataViewCtrl(wxPanel* parent, unsigned int nPanel, unsigned l
                                             wxALIGN_NOT,
                                             wxDATAVIEW_COL_REORDERABLE | wxDATAVIEW_COL_SORTABLE);
 
+            m_ctrl[1]->AppendDateColumn("date",
+                                        MyListModel::Col_Date);
             m_attributes =
                 new wxDataViewColumn("attributes",
                                      new wxDataViewTextRenderer,