Gets the related menu item for a dataIndex
@private
@return {Ext.grid.header.Container} The header
function(dataIndex){
        var view = this.view,
            header = view.headerCt.down('gridcolumn[dataIndex=' + dataIndex + ']'),
            menu = view.headerCt.getMenu();

        return header ? menu.down('menuitem[headerId='+ header.id +']') : null;
    }