Generates the HTML to be rendered in the injected checkbox column for each row.
Creates the standard checkbox markup by default; can be overridden to provide custom rendering.
See {@link Ext.grid.column.Column#renderer} for description of allowed parameters.
function(value, metaData, record, rowIndex, colIndex, store, view) {
        var baseCSSPrefix = Ext.baseCSSPrefix;
        metaData.tdCls = baseCSSPrefix + 'grid-cell-special ' + baseCSSPrefix + 'grid-cell-row-checker';
        return '<div class="' + baseCSSPrefix + 'grid-row-checker">&#160;</div>';
    }