Adds the specified xtype to the {@link #whitelist}.
@param {String/String[]} xtype Adds the xtype(s) to the {@link #whitelist}.
function(xtype) {
        var me = this;

        if (Ext.isArray(xtype)) {
            Ext.Array.forEach(xtype, me.addXTypeToWhitelist, me);
            return;
        }

        if (!Ext.Array.contains(me.whitelist, xtype)) {
            me.whitelist.push(xtype);
        }
    }