Disables this ActionColumn's action at the specified index.
@param {Number/Ext.grid.column.Action} index
@param {Boolean} [silent=false]
function(index, silent) {
        var me = this;

        if (!index) {
            index = 0;
        } else if (!Ext.isNumber(index)) {
            index = Ext.Array.indexOf(me.items, index);
        }
        me.items[index].disabled = true;
        me.up('tablepanel').el.select('.' + Ext.baseCSSPrefix + 'action-col-' + index).addCls(me.disabledCls);
        if (!silent) {
            me.fireEvent('disable', me);
        }
    }